Strange "flange" feedback question

I use Sonar Producer 4.0.2 with a PreSonus FirePod on a HP laptop (1.75 GHz Pentium M, 1GB, recording into firewire external drive 280gigs, using WDM at 10 msec (FirePod set at 4msec)). Recording and playback work fine. However, when not recording or playing back, when I listen to instruments through Echo Input, a flange-sounding feedback occurs at irregular intervals. Once it starts, I have to turn off the audio engine and turn it back on to make the feedback go away. Any ideas what causes this?

Flanging comes from hearing two (or more) copies of a signal a few ms apart. Most people start becoming aware of flanging [sometimes called 'phasing' by those who've spent time with guitar pedals] in the 10-15 ms range... above 30 or 35 ms one tends to begin hearing two separate sounds (depending on the nature of the sound, of course... a string pad might not be obvious, where a percussion hit would.)

The flanging sound I'm getting isn't a straight flange; it tends to have a beat to it (more, less, more, less, MORE, MORER, MORERER), so there's definitely some kind of feedback loop going on too. And it's bizarre--when transport is running (either to play or record) I don't get it. (At least not yet.)

To me that sounds like digital feedback from a loop. Sounds like you have a input monitoring loop happening. Does it do it with input monitoring off?....................JDW