Hi pitched hum from TT pre-amp?

Hello,

I have a Sonic Frontiers SFP-1 pre-amp. My rig is a Scoutmaster table and a Benz Micro Glider (MC). While playing a record last night, between songs I heard a relatively hi-pitched tone; probably a bit below the 1 Kilohertz frequency (test tone). I gently tapped the top of the TT pre-amp and the tone stopped! The pre-amp contains 4 tubes; two Sovtek 6922's and two 12AT7A's. Do you think I have a tube on its way out?

Any feedback would be welcomed.