Mesa Boogie Heartbreaker popping issue

[jetcode]

Bought this amp from Guitar Center and had to re-tube it and replace both the power switch and standby switch. The amp must have seen a lot of use. Put a pair of CV4004 in one channel and a pair of RCA grey plates in the other. A '59 Amperex BB long plate for phase splitter. The tone is fantastic. Haven't replaced the 5AR4 yet. Every 30-200 seconds or a so a pop will happen and it doesn't appear to be preamp tubes since it occurs regardless of which channel is selected. I haven't tried to replace the BB yet to see if it is producing the problem. What else could be making this sound happen?

Thanks,
Joe

[jetcode]

Forgot to add that I replaced the power section with NOS 6V6GT (Fisher).