FST:
If you have not already, get yourself a piece of 3/8 fuel line rubber hose, and CAREFULLY listen in and around the motor while it idles. Don't be afraid to put the end of the hose right on parts of the engine during this process.
The hose should funnel the noise right up to your ear, and let you pinpoint it a bit better.
One more caution: Be careful about getting the hose too close to any source of vacuum while it is close to your ear. If you are like me, I cannot afford to have what little brains I have sucked out. LOL
Don