Intimitent leaking - Zanussi washer dryer?

After recently having new plumbing installed, our Zanussi washer dryer now leaks from the waste pipe when on the drying cycle. It doesn't leak on the wash cycle. It seems to bubble up over the top of the waste pipe (the pipe from the machine is in an upright pipe within a cupboard which also links onto the sink and dishwasher.). It seems to have done it this morning on a spin cycle too (unless it was the hand wash cycle and I didn't notice). The plumber who installed the new pipe work says its not that. But the local repairman said its not the machine (over the phone). Help!