Boss ME50-B Hiss

[GTBass]

Hi, anyone out there with any experience of this problem?

My ME50-B has developed an annoying 'after' hiss. When holding the guitar it is completely noiseless but once a string is plucked a hiss accomapnies the note, decaying with the signal. It is extremely annoying. Unfortunately the noise reduction knob seems to do nothing to cure the problem. It has been fine up until now but the last few outings with it have been a real pain. Nothing in the rig has changed. With the band in full flight it is less noticeable out front but in those, once, quieter moments it's toe-cringingly horrible. The hiss occurs when no effects are selected (on playing a note) and doesn't respond to EQ changes, but is particularly bad when using the phaser or chorus.

If anyone has any suggestions I would be most grateful. I've tried different venues, guitars, pups, and amp combinations all with the same result. Taking the unit out of the chain removes the problem. I can only surmise the ME50B is picking up something but I've come to a dead end in finding a resolution.

My thanks in advance for any advice forthcoming that might help kill this problem and preserve my sanity.