I'm a bit confused by the suggestions that you mess with the tube sockets -- the process of removing the original tubes and putting in new ones should have taken care of any issue that was caused by corrosion build-up or a poorly seated tube. Next, the odds of a different set of tubes exhibiting identical problematic behavior is unlikely (though not impossible.)
If the volume control, you'd almost certainly notice the problem as you turn it up or down.
Once you've toyed with everything and re-set the tubes and all other cable connections (not forgetting to plug/unplug at the amp end) the last thing I'd suggest is to disconnect the preamp from the amp. If the amp by itself continues to make noise, it's not the preamp causing the problem.
If the amp is quiet, plug in the preamp, but with no sources connected. If at this point the noise returns, it's time to take your preamp over to the shop for a professional look.