Tube preamp noise

Many thanks.
I will try to find a bad tube. Well, one of my friends is a strong bias on tubes and almost forced me to buy a tube preamp when I was about to upgrade, but I am not new to tubes. I may be forever like that. ;-(

Anyway, thanks for your help. My preamp uses 6SN7.
I will try to find a bad tube and replace with another, and see if the problem goes away.

Thanks,
I may contact VTS later. The preamp is 4~5 years old and uses 4 6SN7s. It came with 4 sets of 6SN7 GT, and 2 GTAs and 2 GTBs. All tubes generated microphone tapping sound on tapping with the eraser end of a pencil. Small bumping sound appear to be OK but I found two GTAs are much worse than others. The GTA and GTB sets generate much worse cracking (or whatever) noise than 4 GTs. Among 4 GTs, one seems to generate more noise.

Furthermore, among 4 sockets, one socket always generates more tapping noise regardless whatever combination of tubes used.

I mixed (those seemingly good) 3 GTs and 1 GTB, and the tapping noise got reduced a little bit, but now there is more hum noise. Is it OK to mix GT and GTA/GTB?

I guess for many tube amp owners such problems are not uncommon, and hope my preamp is not defective other than tube problems.

Furthermore, among 4 sockets, one socket always generates more tapping noise regardless whatever combination of tubes used.

This might also indicate that a tube socket needs to be replaced.