Funny sounding Passat 2.0L Sport

Hi

My brother has a 2.0L Passat Sport (01) which when revved makes a sucking sound, not unlike the sound you get when you leave the Air Filter out.
I have had a look and can not see anything obvious, the sound eminates from the right hand side of the engine bay as you look at it.

Any ideas?

Thanks in advance

Loose/split vacuum hose?

In the 2.0 NA there's nothing on the right hand side of the bay except hoses.

I'd go for a leaky vacuum hose too.

I checked as far as possible when I changed another hose on the inlet manifold, there is one hose that runs down underneath the inlet manifold which I can't get near, is this the one you mean?