Knocking noise at dash and steering wheel

[A&E]

Help in problem of a knocking noise coming from the engine. The noise is a knocking sound with a vibration translated to the dash and steering wheel.
This noise is present when the car is in gear and idling.
My guess at first was a engine mount in the back of the engine. It was checked and the mounts are good.
I was told the heat shield can be the problem or again the engine mount at the back tend to sag.
Someone have this vibration and noise coming from the steering wheel? Its a 92 Passat GL automatic transmission. Also the noise is not always there esspecially when a mechanic is looking for it. Any suggestions, or should i start to do unneccessary work that might not be the problem?