Yeah - that's where I'd put my money if I were betting. If you have any deoxit on hand, try that on the pins. If not, and they look oxidized at all, you can also try sanding the pins or repeatedly inserting/pulling them as a means of cleaning them off. Folks recommended that I do this a year or so back with a 1950's rca blackplate 12bh7 I had that was humming. It solved the problem.
FWIW, my finding has been that price is definitely not a guarantee of a great tube. My favorite input tube at present in my crack is basic tung sol 12au7 that I bought for about $10. I periodically compare it to pricier mullards, amperex and rca's sitting in my closet, and keep coming back to the tung sol. Picking up a bunch of cheapies on ebay is, imo, a better way of finding one you really like than dropping a bill on a single 12au7.
