odd phantom rattle noise, 99 YZ400f.

I have a strange rattling noise that i can only duplicate while riding. It is on the right side, down low.  It almost sounds like the chain slapping up and down, but i can see that chain and dont think thats it, plus the chain isnt excessively loose. It happens at lower rpms, on harder acceleration, for example if im in 2nd or 3rd, and slow down for a turn or something but dont shift down, and accelerate out of it, and it usually is only for a couple seconds. Cannot hear it when idling or at high RPMS. I know my description is horrible, so i apologize for that. Bike runs really well.

How many hours ago were the piston/rings replaced? Can start getting weird rattles when the rings are starting to go...Bottom end bearings are good? Could always be something weird, I had a weld bead bouncing around inside my frame, had the ENTIRE bike apart (for another reason) lifted the frame to carry it outside and wash, and Ting Ting Ting, sounded like a damn marble bouncing around in the frame, was super hard to pinpoint because it resonated through the whole frame...Sprayed some foam into a little hole to trap the rattling piece, and walla, that weird rattle I've always heard but couldn't figure out was gone, haha...

How many hours ago were the piston/rings replaced? Can start getting weird rattles when the rings are starting to go...Bottom end bearings are good? Could always be something weird, I had a weld bead bouncing around inside my frame, had the ENTIRE bike apart (for another reason) lifted the frame to carry it outside and wash, and Ting Ting Ting, sounded like a damn marble bouncing around in the frame, was super hard to pinpoint because it resonated through the whole frame...Sprayed some foam into a little hole to trap the rattling piece, and walla, that weird rattle I've always heard but couldn't figure out was gone, haha...

The piston and rings are fairly new - im guessing around 20 hours? 30 at the most? As for the bearings, i have no idea. ive never had the case split since ive owned this bike, only been a 3 or 4 years though

My guess would be the crank arm bearings are worn (number 5 in the parts diagram). https://www.thumpertalk.com/shop/oem.php?partcategory=901496&manufacturer=7&category=5&year=1999&model=28998

When you start getting play in the crank arm it can sound a bit like piston slap...

If you've been riding it fairly regularly for 3 or 4 years, it may be time to do the bottom end, ESPECIALLY if it's still on it's original crank/bearings...

But man, hard to say, if you say it sounds like chain slap, maybe it is?!? You'll definitely get a little happening here and there even with everything adjusted correctly...If you can't get the bike to mimic the noise while revving on a stand for a few seconds, maybe it is? Or even just something silly like the chain guide has worn, is a little loose and is slapping the swingarm? (a lot of people will put foam or tape under there to prevent that)...So just rule out the simple stuff, and keep going until you end up checking the freeplay in the rod...

My guess would be the crank arm bearings are worn (number 5 in the parts diagram). https://www.thumpertalk.com/shop/oem.php?partcategory=901496&manufacturer=7&category=5&year=1999&model=28998

When you start getting play in the crank arm it can sound a bit like piston slap...

If you've been riding it fairly regularly for 3 or 4 years, it may be time to do the bottom end, ESPECIALLY if it's still on it's original crank/bearings...

But man, hard to say, if you say it sounds like chain slap, maybe it is?!? You'll definitely get a little happening here and there even with everything adjusted correctly...If you can't get the bike to mimic the noise while revving on a stand for a few seconds, maybe it is? Or even just something silly like the chain guide has worn, is a little loose and is slapping the swingarm? (a lot of people will put foam or tape under there to prevent that)...So just rule out the simple stuff, and keep going until you end up checking the freeplay in the rod...

it only does it when the bike is actually moving, and under load.

it only does it when the bike is actually moving, and under load.

definitely not chain slap then, that usually happens braking into turns, and when bouncing around at steady throttle, not so much when pulling hard...Maybe just pull the whole head and see if your crank arm has too much side to side play in it?

On the contrary, YZF's are well known for producing a chain slap under low rpm lugging conditions.  It has a sound more like slapping a board against the swing arm than a metallic, ringing noise, and a properly adjusted chain will definitely do it.  

 

But YZ400's and the first year 426 had a rattle they would make that is caused by the balancer shaft drive gear loosening and rattling around on the square key that anchors it.  Normally, though that would not be load sensitive, only speed.  

Gray, that is a great description of the sound - slapping board against the swingarm. I honestly dont know if its properly adjusted, i know its definitely not too tight, and its not so loose it falls off, so i havent worried much about it.

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