I stand corrected - I heard that very same noise last night. However, I only hear it while the QC is booting up. Once the boot cycle is complete, and the QC starts making output, the noise goes away. No idea what that means. The weird thing is, I did have XLRs (outs 1/2) connected to grounded monitors at the time and I heard the noise on outputs 3/4. (The QC and monitors may be grounded differently than the power amp on the 3/4 outs. Not an issue once up and running, so not really worth "troubleshooting", i.e. no trouble...)