calling the experts - B15 making funny noises...

I was playing my B15 today and it popped on me. It's done this once before, but nothing happened after the first time so i wasn't too concerned. I had the amp running for about 6 or 7 hours while I was in the studio the other night and then it spent a day in my car before finally being brought back into the house. This time after it popped, I hit the stand by, then powered it off. I fired it back up a few minutes later with no issue.

As for the amp, I got it a few months ago from a local fellow who had owned it for quite some time. It still has the original ampeg 6SL7's, JJ 6L6's, and a really old looking Stromberg-Carlson 5AR4. I haven't opened it up, but from what I gathered when I bought it, it hasn't had any repair work done.

I'm taking it down to get checked out tomorrow, but I was hoping I would get some suggestions as to what specifically it could be.

Thanks