My 97 NAS D90 seems to have a hissing sound coming from the engine bay. It's noticeable at idle, and seems to get really loud if I'm in drive. If I'm in neutral or park, it's much harder to hear, so harder to troubleshoot without someone to help.
The hissing sound will "warble" as the engine warms up or if idle is bouncing around, like at a stop light after high revs. If I increase the throttle to say, 3K or more, the hissing sound goes away almost entirely. And not because it's being drowned out by a big V8.
My guess is that it's a hole or vacuum leak around the throttle area, but I'm not too clear on how this system works. There seems to be some vacuum lines running to the MAF sensor in front of the throttle body, but I don't understand what they're doing. Is this to warm up the air before it goes through the intake manifold?
My local dealer had fixed this problem in the past once, saying that it was a split hose, but failed to show me which hose it was and now they have no idea what the problem is since the original tech that fixed it is now gone. Any help is greatly appreciated as the sound is deafening at idle and has prompted friends to start calling my Rover things like "Slitherin" and other dorky snake puns.