itasha

it seems to be coming from the right passenger side of engine. makes a vibrating sound I'm not sure what it is, subsides after 40k rpm for some reason

sixshooter

Open the hood and rev it by hand at the throttle butterfly and see where the noise is.

Jerry L

The right passenger side of the engine? Hmmm... I never tried carrying passengers there. Might be fun. :-)

If you don't have a mechanics stethoscope, an old trick is to use a long screwdriver. Place your ear on the end of the handle an place the other (sharp) end on various places on the engine while it's running. I shouldn't have to say to be careful of moving parts, but I will.

Good luck!