Hissing noise from engine bay

Hi everyone, I wonder if someone might be able to assist me? My range makes a hissing sound when the engine is running, sounds like air is escaping from somewhere; it only makes the noise when the engine is running and stops every time I use the throttle but then starts as soon as I release the throttle. It doesn't seem to be affecting anything ie suspension etc. But, I am concerned it could be the start of something BIG and EXPENSIVE.
If anybody can shed some light on this problem, I would be very grateful and relieved.

The range is a p38 4.0 HSE 2000 X reg.

leaking fannymold gasket?

Cheers Mr Hat Man, I'm complete novice; where would I find the manifold gasket?

reg3254 said:

Cheers Mr Hat Man, I'm complete novice; where would I find the manifold gasket?

Click to expand...

Between the manifold and the cylinder head. The inlet manifold is the one you can touch with engine hot, the exhaust manifold is the one you touch and leave skin attached to.