Possible buying a used amp

Hi Skip,

I checked out a used tube amp today and it sounded real nice. The thing is after awhile it started to ring and when you tapped the amp, the ringing stopped. It happend a few minutes later so the seller pulled a tube and replaced it. I did not hear it ring again. I know without seeing the amp it is hard to tell what might be going on but does that ringing mean a tube could be going bad and tapping it would stop it or more likely something with the socket.
I am still going to buy the amp but wanted to know if this in your opinion is not a major issue or it is going to cost me more dollars to send it out if it happens frequently.

Thanks 528hemi