1996 TRX200D clunking noise

So recently bought a 96 TRX200D for my son as his first atv. It was in mint condition, never ridden hard and taken care of by a guy I know who is very into maintenance. It ran great, my son rode it around the yard a few times to get used to it and everything was good. We took him and the TRX200D out to a off-road park to let him play around in a little mud and a small creek and some little hills. Still everything seemed fine.

He got it out the other day to ride it around the yard and I noticed it is making a clunking/ tapping noise. It sounds like it is coming from the right side of the engine case as I can feel it a little in my right foot on the peg. This is in neutral by the way, it does it while moving also. It is defiantly somehow connected to the speed of the engine because the frequency seems to change with engine rpm.

I'm nervous to run it for long and haven't let him ride it since. Just looking for ideas on where to start before I completely start tearing down the motor.

I will try to get it on video tomorrow to see if that will help.