ive had this problem too mate, i own the efx500 and have had it linked up to my mates pcv275 and we got the same sound, like a hi pitched flangey / jet noise. and ive also had the same problems when i linked it up to a denon dnx1500

sadly i dont know how to combat it, we had to unplug it and have the efx between mixer and amp, meaning we couldnt effect one channel at once,

really would like to know how to combat this problem

I've kept playing around with it. I HATE having the efx between the mixer and the amp. I found if you adjust the effect send and recieve levels at the top of the efx500 you can almost get rid of the noise. Try turning the send level to about 730 and bring the effect incoming level to about 130 or 2o'clock. tell me if that works better for you.