Cracle in one channel

I have a TAD60 and experience a momentary crackling sound in one channel. It goes away and usually does not happen when Ireseat my power tubes before I turn them on...I ust cleaned the contacts with rubbing alcohol a couple of days ago. Any ideas?
Sounds like a flaky tube. Swap the tubes left to right and see if the problem moves to the other channel. Don't forget to bias the amp after the swap and also after any tube replacement.

Sorry for the poort spelling...

Thanks Reubent. Turned out that one of the RCA 12 AU7s were noisy. I replaced with Amperex and unit is quiet again...
Great news!