I have swapped over the 2 vacuum transducers, the boost does seem to be slightly smoother which is good news, however it still screams on boost.

I will be taking the inlet manifold off to clean everything on Monday, hopefully its just a case of a leaking seal somewhere giving me a boost leak.

Just having a think as well, although the noise is located on the EGR side of the engine, is there a way I can test the viscous fan to make sure that its not that which is making the noise as the engine spins up etc?

I would look at:

The junction of the aluminum up pipe to the EGR "mixing chamber". The mixing chamber is the cylindrical piece on the bottom of the intake manifold that has a diaphragm on the side of it and also has the rubber line that runs to the MAP sensor.

The mixing chamber has two places it could leak. One, at the o ring where the up pipe joins, the pipe is secured by a bolt at the rear of the junction and the boost pressure can force the two pipes apart at the front. Two, the junction of the EGR pipe leading from the exhaust manifold to the side of the mixing chamber can have a leak.

I've been experiencing the same 'whistle' at the same lower rpms, 1800-2000. I have nicely cracked CCV down pipe to the cold air intake on the turbo. I can say that I had the 'whistle' before and still do with no change in its tone or where it happens in the rpm range. I will be replacing the fossilized rubber bend in the next few days and report back.

What has me puzzled is that it only happens at the lower rpms=low boost. One would think it would get louder as PSI increases with boost. Mine does not, which I think rules out a bad intake to uptube oring or egr connection at the same location.

Hoping there is a solution too.

I am going to be cleaning the inlet manifold soon anyway, I can tell that the manifold has been off before as I know that all of the fuel lines to the pump have been refreshed. I am guessing that whoever did it previously might have either damaged or not replaced old seals on the EGR side.

I am however going to try and get my hands around that area to check for any connector that has become loose over time - needed to wait for the engine to be cold before doing this!

There is very limited space on that side of the engine but I can see that there are bolts pointing upwards (rather than being tightened from below) from when it was last taken off.

I am going to check for leaks and try and tighten up everything first before pulling the manifold - it will need cleaning but I just need the car to be running as best it can before I do a few drag runs in it.

The EGR and manifold come of as a unit. Don't try to remove the 4 bolts holding the EGR to the manifold.

There is one single bolt in the rear of the connection, accessed from the bottom of the car with about 16" of extensions.

I have a feeling that this single bolt has been done up from the top side rather than from under the car as it would have been done originally. I am going to check everything is tight and try and inspect anywhere I can see an obvious blowing of air/muck.

I know there is also the MAP sensor vac/boost line in there as well, will check to see that it is also air tight. I have seen photos of the inlet and EGR together, I believe they use a female TORX bit vs the male T40 bits for the inlet manifold bolts.

I am hoping that I can just check the connections on that side for leaks and not need to take the manifold off initially. I will be looking into porting and gasket matching in the future though.

Fingers crossed its something really simple!

Edit - after checking out the glowplug change guide which has the inlet manifold removal I have a feeling that it might be the intercooler to EGR O ring that might be leaking as I know its quite messy around the pipes in that area which might mean that the noise I can hear is the O ring sometimes sealing and sometimes leaking. I am also guessing that the leak might be quite small owing to the fact I am getting boost, although some of the time it does have an issue intermittently.

This is exactly where I was talking about.

On my car, boost pressure had bent the inlet tube backward causing it to come loose from the o ring. The previous owner/service person had employed a redneck/bogan solution by drilling a hole in the front of the EGR, along the rim and threading a screw through, into the pipe to hold them together from the front.

It has worked so far and I've left it alone but I plan on updating it in the future when I remove the mixing chamber in favor of a straight pipe.

Ok, I have been out and I have found the issue, well there were 2, the first was the air temp sensor just below the EGR, the O ring had gone solid and 1 of the clamps was broken, I have replaced the O ring and reseated it all.

The main boost leak is actually...a hole in the intercooler, it looks like there has been a bandage/patch on it to fix the leak however its still leaking badly. I am guessing that at speed this patch is held tighter onto the intercooler and the boost is kept in mostly vs around town where is just leaks.

I am going to get a replacement intercooler, unfortunately I don't have the insurance to replace it with an aftermarket one so I am going to get a used OEM one. I might also try and temporarily fix the current one in situe - need to find out what I need to do to remove it.

Pretty easy to remove.

I noticed that it thankfully at the very front of all of the radiator/coolers etc (guess its probably got holed due to a stone or something).

I can see there are 2 bolts on the underside - are those the only bolts holding it onto the car (apart from the pipework attached at each end?

IIRC that is correct.

I have removed mine as well and it seemed pretty easy and intuitive. I seem to remember having to work a little to pull off the rubber hoses and once those are free there was a couple bolts and out it came.

Thanks guys, usually when working on other cars you see the 2 obvious bolts that you assume is all that is holding it on, only to find that is a hidden hard to reach bolt which requires a day's work to get to!

I have another intercooler on its way, might try and remove the stock one tomorrow, although I might just work on doing an oil change or fuel filter change instead and wait until I have a part to fit.

Will be interesting to find out the sort of power the car has, pretty sure it wouldn't have had full boost with such a leak.

That fortunately does not seem to happen too much on this vehicle. It was the first (or maybe the second, the 140 may have this honor) model that MB completely computer modeled during development. Everything just seems to fit together in a logical manner.

It will definitely fly once the boost builds. Best technique when you need acceleration is not to floor it, but push down about 1/3 to 1/2 way and give the boost a chance to build. Then look out, you will be doing 80 mph (150 km/h for you guys) before you know it...