1.8t20v When driving strange groaning sound ???

[Turbo-Tyson]

when driving my golf turbo (x reg) at a certain revs there is a strange groaning sound almost like a air leak or something . Could it be a faulty wastegate or air getting forced out of somewhere ? any help would be great thanks .P.s Its done 114,000 miles

[Rhyso]

what RPM does it happen at? does it get louder the faster you go?

[Turbo-Tyson]

its weired it only does it at at certain revs in different gears but it doesnt do it hen it is stationary , do you think it may be because i have a single piston dump valve at the back of the engine bay ? iv been told that having a single piston messes up the performance and a baileys recerc valve will sort it and let it run alot better , iv got a bosh plastic 1 but i think its broken as when you pust the diaphram inside and put your finger on the vac pipe nipple there is no suction there . Hope somebody knows or itl be a trip to a&b garage next week