Odd Knock

I have a 91 with a 350 and I have an odd knock

I only happens when the air temperature is in the Low 80's and below, then it only happens at idle. It only happens when the engine is up to temperature, Then of you turn on the a/c or put and other load on the engine it will stop for a while.

Any ideas?

I doesn't affect performance it's just really irritaiting not knowing what it is because short of rebuilding the engine I don't know how to get rid of that horrible sound. And I don't have the time or money to do so right now.

I don't know if this will help but when it was really cold out my 97 would sound pretty crappy at start up so I switched to synthetic oil and it seemed to help out in my situation.

Sounds to me like a possible cam bearing... older 350's were bad for it