jcm900 effects loop problem

[n pomares]

i want to use my maxon ad9 delay pedal in the effects loop of my jcm900 where i think it sounds better than into the main input. i rewired my pedal board so that all the rest of the effects are in the main input as always and the delay is in the loop.everythig is fine until i use the cry baby wah. when this pedal is activated there is a whistle or a howling type sound that changes in pitch with rocking of the pedal. if i remove the delay pedal from the effects loop the noise is gone. has anyone ever heard of this type of problem before? any help is greatly appreciated.