Whooshing sound....

[seanm]

While at practice tonight I started getting a whooshing sound from my rig. I put a Boss TU-2 in front of the preamp and that seemed to solve the problem. However, the TU-2 had a very low battery and went dead. It "ran" about 2 hours with the battery too weak to light the LED but then started making noise. When I plugged straight back into the preamp everything was fine.

I couldn't try anything since I did not want to disrupt the practice. Some info:

• Preamp design here: http://seanm.ca/stomp/preamp.html
• I use a cheap 24V wall wart that produces 30V under load from the preamp
• I use an LM7824 to regulate the voltage down to 24V.
• The ground wire in the basement is floating 
• I don't think heat was a problem since the preamp felt cool to the touch.

I am not sure the problem is the preamp. Thoughts?