DL-4 hum/buzz when half-speed overdubbing?

[Travis Hartnett <tiktok@sprintmail.com>]

The other day I noticed that my Line6 DL-4 now includes an unpleasant 
buzz/hum/whine when recording in half-speed mode.  Switching back to 
regular speed just shifts it up an octave, although recording and 
playing back at full-speed have no problems.  So, I pulled out my spare 
(neither one has seen much use recently) and found that it had the same 
problem, although to a lesser degree.  Both units are about three years 
old and have seen only light usage and been stored on a pedalboard or 
somewhere safe, and I've tried different guitars, amps, cable, etc.

I know they didn't used to do this--has anyone else seen this problem?

TravisH