I have a 2003 sportsman 400. When I ride I hear a loud whistling noise and I can't seem to figure out where it's coming from. When I increase throttle at about 30 mph it is very annoying. When flat out, it's gone. Also, no noise when idling and up to 20 mph. The transmission was re-built by a local dealership a year ago. Someone told me that the gears are not aligning right which causes the noise but not sure. The problem is gradually getting worse. Anyone with any ideas. Thanks in advance.

 

does it only when in gear and not in neutral when you rev up the engine? just trying to rule out the belt and clutches. did you check the tranny fluid as well? as far as the gears not meshing it would almost sound like there would something out of alignment when assembled? just a guess though unless something is worn enough(like the cases) wear its letting it out of alignment. there again this is just a guess.

 

Tranny fluid is the first thing I checked. It's just the way it's suppossed to be. Also, I checked in gear and neutral and no whistling when reving in neutral. I only hear when riding, mainly around 20-40 mph and only when throttle is engaged. If I'm doing 40 mph and let off the throttle, the noise disappears. When i hit the trottle again, it's there. Thanks again for your help.

 

My 2002 500 has done the same for over a 1400 miles, I think its wind noise which does not seem to be a real reason but the only 1 I can think of. I have had it in the shop but they could not find anything wrong. Runs great and no metal in the oils when I change them. After running through a snow drift then back on dirt the whistling was gone for a mile or so until the snow packed around the fame melted also with a very strong wind blowing from the side its gone which is why I think maybe wind noise. Almost 4000 miles and still no problem but this noise has never gotten louder,only changes with speed same as yours.

 

Same here with oils and no metal. Also it's runs great. Except for the whistle, I have no problems. I'll keep a closer eye on the wind as I certainly never thought of that. Thanks.

 

Could also be a pin hole or crack in any of the intake manifolds or even in the air flow tubes for the auto clutch.

You can buy a mechanics stethoscope to help pinpoint the source

GA111D, Stethoscope, Mechanic’s

 

But if it's a crack or pin hole and I rev it up in the garage I would think it would show up. I have tried to reproduce the sound in gear with the ATV on a ATV jack but only happens on the road. I keep thinking about using a piece of garden hose and taping it to different spots under the quad then holding the other end to my ear as I ride.
Bet that would get some laughs on a trail.