ringing bell in speaker

[radio nut]

I am currently working on a General electric model 810(Sams set 53 folder 12 date 1/49). If I turn the contrast to the middle the speaker has a low pitch rumble that stays. Here is the interesting part. If I tap the tubes or chassis with the plastic handle of a tool the speaker gets a bell type sound as I tap and it goes away. It happens if I tap anything, caps, resistors, coils even the workbench! This set is fully recapped, the resistors were checked in the set. Sound goes through it okay as long as the contrast is turned up past half, below that there is no sound. Please, what do I need to check!!