A couple of gremlins

I'm convinced it's a vacuum leak near the rear left side of the manifold area - but the vacuum in the manifold definitely decreases as you open the throttle, as it allows more air in :) so the noise should get quieter.

It could well be the PCV gasket, the crack in the manifold (hope not though) or any of the little vacuum pipes round there. As Jetset says, take the cover off and just have a damned good listen and spray brake cleaner or something around the area. I'm sure you'll find something.

EDIT: Oh, sorry Jetset - just seen your edit!

Hmmm - well I hope he's talking about the exhaust manifold then because the inlet manifold is the one with the vacuum!! and is the one that will hiss when it has a leak. It's the exhaust manifold that will blow and be quite loud if it's leaking.

Also, the problem with our engines is that they are so torquey and monstrous anyway that even if there is a problem and not running on full power it's actually quite hard to tell! The only time I've ever noticed a true difference in performane was when I renewed the spark plugs and it seemed like a different engine and felt so responsive and so much more potent at high revs.

Back to topic though...! (Sorry) I'm sure that your seal is good, just throwing out possibilites! It could be something completely different, but just from everything you've described I think the first thing you should try is to test for leaks around that rear left area of the air intake manifold and all connected lines. Once that's done and you can be sure it isn't that, then it'll be easier to let it go and think of other things :)

I'm still a bit concerned that you've been told by a mechanic that it's very unlikely, because most people will tell you that a hissing sound from the engine bay of a V8 BMW is MOST likely to be the inlet manifold gasket because they are prone to going. And considering that's just been tampered with it's the first thing I'd have thought he would think of!! Anyway, I hope you find something tomorrow - good luck!

I have heard vehicles run with inlet manifolds off and the engine is a lot louder. If you look on realoem.com, the intake is actually called the intake 'muffler' it smoothes the airflow going into the cylinders and also covers the actual part the air is sucked in past the valves. This process is quite loud and the intake manifold does muffle the sound. If there was a significant leak, the engine would be louder and I'd be losing a lot of vacuum causing all sorts of fuelling issues.

This is why my thoughts are simliar to jetset's, I'm thinking either a vacuum pipe is leaking (though I did renew some of these when replacing the PCV) or a pipe is possibly trapped. One thought that just ocurred to me now.... there are two pipes coming off the PCV valve. One one the drivers side that goes onto a fuel pressure valve on the rail and on the passenger side it goes off down under the nearside somewhere (passenger footwell area) The pipe on the passenger side was totallyperished and would not have survived being re-attached so I took a normal length of vacuum pipe and used that. When I took the old pipe off, it looked like there was a tiny little piece of plastic tubing half way down the pipe with an internal diameter of about 2-3mm. The mechanic said it looked like a joiner which must have been used previously and had broken off leaving a piece in the pipe. My thought now is, could this have been some type of restrictor that was meant to be there? if the vacuum has to suck through a smaller gap, it makes it more powerful does it not? My theory is, could this be causing the vacuum to be sucking too much and causing an imbalance?

It's only an aside theory but wondered if anyone else noticed this in their pipe?

Didn't manage to look on Saturday. I have been using a work van so not had the car either. I'll do the brake cleaner test tomorrow.

Mike

Ah right, I see you are talking about something completely different - the intake manifold is not a muffler but simply (actually quite complicatedly!) directs the airflow into each intake port. As said, without this the engine would simply not run - if it was the intake manifold (and not the silencer or air filter housing) that was removed from the engine that you heard then it was either a diesel or it had individual throttle housings (an M5 perhaps...?) or individual carburettors. The only additional noises you will get with an open intake system are a slight sucking/hissing noise from the throttle valve and induction noise when at half throttle or more. My old motorbike used to give off a little valve-tap as well but I don't think that would be noticeable on a car.

My thoughts are are also similar to Jetset's still and all of the vacuum lines around that area become very week after 15 years of service. Let us know how the brake cleaner test goes.