Tube preamp noise

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.

You might buy some new production 6SN7 tubes...like Shuguang...and install those. The new tubes should not be microphonic, especially if they are purchased from a dealer that screens for this problem.

If the preamp has microphonics with the new production tubes, then you'll know you have a problem with the preamp and not the tubes.

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.

Socket replacement? It is an easy job?
How often a socket can go bad? Is it a common problem for tube amp/preamp?

Thanks much for your help.

"You might buy some new production 6SN7 tubes...like Shuguang...and install those. The new tubes should not be microphonic, especially if they are purchased from a dealer that screens for this problem."

I got Shuguang 6SN7, and yes, I found they do not suffer from the microphonic problem. From new fresh out of the box, they do not sound as good as Sylavania or GE, but at least I am glad to find out the problem wss due to old/bad tubes, not to the sockets or other parts of the preamp.
Thanks!