"Thumping"sound from piezo?

Got a call from a cousin of mine re his Crafter electro-acoustic.When plugged in but not being played there is a "knocking" coming from the guitar,quite clear.He contacted Crafter who sent out a new preamp/control module but the problem wasn't solved-good customer service though.I've not come across this before but we both reckon it must lie in the undersaddle element?
Anyone have any suggestions or experienced this problem?Thanks!

Are you saying that without any provocation, there is a 'thump' coming from the pickup?
IF so, is it rhythmmic? Random?
I am going to suspect something other than the pickup and preamp. Is there a drunk little leprechaun sneaking behind him and thumping on the guitar? LOL

By saying this is happening "when not being played", do you mean while it's sitting on a stand with no one touching it? Or do you mean when he stops playing, but is still holding the guitar?

Some of the piezo elements out there are extremely sensitive, and will thump, bump, knock, etc. with even very subtle handling noise. The slightest movement or touch to the guitar's body can make quite a racket.

Is there a drunk little leprechaun sneaking behind him and thumping on the guitar? LOL
That hasn't been ruled out yet!...but hes a sensible kid:wink:so I'm hoping not!
I wonder though is it any form of electro-magnetic interference,he has a lot of gear in his den....

Jake the strange thing is it is rhythmic,its not noticable when playing and its when its plugged into an amp whether held,left lying etc- could it be related to the house mains electricity?
I'll give him a call soon and see how he's doing,he was planning to take it to tech soon...

Take the guitar to another location, plug it into an amp. IF the thump is not there, the guitar is not the problem. Take his amp with him. Plug the guitar in to his amp. IF the thump is not there, the problem is no with the guitar or the amp.....it is in the environment in which the thump is evident.