Knocking In Neutral

[banjaxed]

I just have had the flywheel, clutch etc replaced. I noticed a thumping, kind of knocking when in neutral with engine running. Seems to be coming from underneath or behind. Not a consistent rhythym to it, thump thump pause thump pause thump thump thump pause etcetera. I can't feel it when driving. Brought it back to mechanics , they can't find anything. Any suggestions. New timing belt six months ago.