Was a bird nesting in it?

That would have been a humming sound me thinks.

Ray, I'll offer one thought.  I learned this, the hard way.

At one point, I put by bike on a front stand.  I did it just to see if my stand (pit bull) was strong enough.

My stock bike had a plug, in the steering stem, bottom side.  To get the peg in place, I had to remove the plug.  Of course, I did not remember to put the rubber plug back in place.

At 70-80, and only there, the bike made a sound as if I were blowing across a bottle top. 

It might be worth checking as you could cover that hole with a piece of tape, just to see if it helps.

Ray, I'll offer one thought.  I learned this, the hard way.

At one point, I put by bike on a front stand.  I did it just to see if my stand (pit bull) was strong enough.

My stock bike had a plug, in the steering stem, bottom side.  To get the peg in place, I had to remove the plug.  Of course, I did not remember to put the rubber plug back in place.

At 70-80, and only there, the bike made a sound as if I were blowing across a bottle top. 

It might be worth checking as you could cover that hole with a piece of tape, just to see if it helps.

Interesting.  I checked (it's just above the front fender?), and there isn't a plug there.  Just a 1 ½ inch hole...

Update: No joy.

So this morning, before it got too hot (Central Valley heatwave), I covered the steering stem hole with some duct tape in hopes that it could be just as simple as rcannon suggested.  (Thanks for the suggestion rc!). Alas, the howling was still there. 

It certainly sounds like its coming up from the front more so than the back, but that could be because the sound has nowhere else travel except the through opening at the forks.

Something else I noticed, and this might help you experienced diagnosers, I can detect a hint of the howling noise between 40-44 mph.  Same thing with the clutch, as soon as I start to pull the clutch in and get into the grey area or completely pulled in, the sound goes away.

I took two phone videos of the noise.  The first vid I just rode up to 78 in 4th gear and demonstrated how pulling in the clutch cuts the sound.  The second, I rode up to 78 and demoed the sound in 4th, 5th and 6th gears, and then tried to video from the front of the bike to the rear.  Unfortunately for the second vid, the wind noise is too great to really tell. 

In the video, the noise doesn't sound so bad, but, from the seat, it is pretty loud.   

Now I just need to figure out how to get the video to you all...

Here's a link to the first video:

https://youtu.be/diUhHyZuCPI

Basically, I sped up to 78, was in 4th gear, and pulled in the clutch a few times.  I stopped recording as soon as I started to slow but you can just catch how the sound goes away when I do. 

Your clutch noise is normal.

Your clutch noise is normal.

Damn!  You think so, JMac?  LOL.

The video really doesn't do it justice as to how loud it is.

Well, a video (+ audio) is worth a thousand words.  I do hear what is almost a "tone" and could tell the short several times you pulled in the clutch starting at 19 seconds (you can see the tach jump too).  Good job and good idea.  The tone is changing pitch with RPM, so this is not wind noise or other such stuff.   I can't tell how loud that "tone" is from the video, although to me it doesn't sound loud.  I have not noticed such a sound on mine before, so I am not going to declare it "normal" at this point; but it could be (plus my helmet is loud AND I wear earplugs/earphones).  Very interested to see what others say.

Your problem is this:  If it is truly related to speed, then it probably can't be the clutch or engine (since clutch and engine both see RPM, not speed).  Those would produce a similar sound at various speeds in each gear.  It could be the differential or transmission, which DO see speed.  With the differential, when you pull in the clutch, although the differential is still spinning, it has no power load on it anymore (the load goes backwards) and that could make a sound from it disappear.

I am perplexed at the moment.  How OLD is the differential oil and have you checked the level?  (Long shot, but hey, just another thing to eliminate).

I kept waiting for an unusual noise and then the vid ended.

Well, a video (+ audio) is worth a thousand words.  I do hear what is almost a "tone" and could tell the short several times you pulled in the clutch starting at 19 seconds (you can see the tach jump too).  Good job and good idea.  The tone is changing pitch with RPM, so this is not wind noise or other such stuff.   I can't tell how loud that "tone" is from the video, although to me it doesn't sound loud.  I have not noticed such a sound on mine before, so I am not going to declare it "normal" at this point; but it could be (plus my helmet is loud AND I wear earplugs/earphones).  Very interested to see what others say.

Your problem is this:  If it is truly related to speed, then it probably can't be the clutch or engine (since clutch and engine both see RPM, not speed).  Those would produce a similar sound at various speeds in each gear.  It could be the differential or transmission, which DO see speed.  With the differential, when you pull in the clutch, although the differential is still spinning, it has no power load on it anymore (the load goes backwards) and that could make a sound from it disappear.

I am perplexed at the moment.  How OLD is the differential oil and have you checked the level?  (Long shot, but hey, just another thing to eliminate).

Diff oil changed with every oil change.  My motor sergeant/friend is obsessive...

Check the exhaust header bolts to head,sounds to me like a possible exhaust leak around that area.That might explain why it goes away when you pull in the clutch,engine goes back to idle.
Just a thought.

Is this sound something started recently?

Check the exhaust header bolts to head,sounds to me like a possible exhaust leak around that area.That might explain why it goes away when you pull in the clutch,engine goes back to idle.
Just a thought.

Thanks for the thought, Joe.  I tightened the header bolts a couple of months ago.  They needed it, but I never detected an exhaust leak.

Is this sound something started recently?

I can't be certain that it started all of a sudden.  I just noticed it while riding.  I asked the previous owner and he couldn't recall ever hearing that sound.

So I just got back from a trip to Mount Rushmore and some of the accompanying areas, Deadwood, Devil's Tower, Craters of the Moon...

The bike performed flawlessly.  The noise was still there from the 75-80 range, but I just got used to it.  The only curious thing was twice, when I had my cellphone on the accessory charger, when I turned of the ignition key, it "blipped" on and off about 3 to 5 times before staying off.  Weird, but it only happened twice. 

In total, 8 days of riding, 3245 miles, and one SORE butt!  Thank God I put a new Stealth seat on her before the trip, or I would've been miserable. 

Have you pulled the Drive shaft to see how much lube is still left on the splines after eight years? They come pretty dry from the factory. That howl sure sounds like gear whine. Just asking?

Have you pulled the Drive shaft to see how much lube is still left on the splines after eight years? They come pretty dry from the factory. That howl sure sounds like gear whine. Just asking?

No.  And I suspect that it has never been done.  I will research the "how to" and do it myself.

Thanks for the tip!

The only curious thing was twice, when I had my cellphone on the accessory charger, when I turned of the ignition key, it "blipped" on and off about 3 to 5 times before staying off.  Weird, but it only happened twice. 

That happens with certain electrical add-on accessories, like LED lighting on the turn signals.  On mine, it will go crazy if I have any flasher operating (or maybe just the emergency flashers) when the ignition is turned off.

That happens with certain electrical add-on accessories, like LED lighting on the turn signals.  On mine, it will go crazy if I have any flasher operating (or maybe just the emergency flashers) when the ignition is turned off.

Thanks Max.  I DID install a solid state flasher and put LEDs in the rear signals.  In fact, today, I just installed the switchback dry/turnsignal lights in the front.  Gonna convert the rears to tail/brake/signals next...

No.  And I suspect that it has never been done.  I will research the "how to" and do it myself.

Thanks for the tip!

Is this a good demo of how to R&R the driveshaft?:

https://youtu.be/aIL_9iEvvIE