What's the cause of the noise?

[H-townbenzoboy]

Hey, I have a little dillema here. My 81 300SD has this thing where it makes a noise when stopping. It happens when it goes into first gear. The noise is a CLUNK. The mechanic says its the CV joint, and it poses no real threat, just an annoying noise. I just want to get a second opinion here. The mechanic has been working on Benzes for a long time, and I trust him, he knows more than me, but I just want to know is it really the CV joint?
-Joe

[BoostnBenz]

Don't trust your mechanic. What you hear is the tranny cramming your car into first gear, it isn't much of a threat to safety but in time it may take its toll on your drivetrain. You have a vacuum leak of some sort, not enough vacuum is making it down to the tranny's modulator (so the shift is harder than it should be). Put a vacuum pump on that line (black line going over camshaft cover) to see if the modulator side is holding, if it is then inspect the other underhood portion (remember the black valve box on top of the camshaft cover, it shouldn't leak, but it is ok if it leaks a LITTLE) if you still don't find anything then you may need a new bleeder valve. (white valve on top of injector pump)

[H-townbenzoboy]

Ok, so if the CV joints needed replacement though, what would I feel or hear? I'm not much of a DIY person, I don't wanna screw things up worse than they already are, since I know very little about repairing cars in general. If it is a vacuum leak, how would it be repaired? And where do I get a vacuum pump from? The vacuum has stopped working in some parts, like the door locks, and the central air vents in the front. I don't worry about the door locks though, but I need my air. The a/c work is almost done, and should be completed very soon.
-Joe

[BoostnBenz]

I doubt you'd feel anything while driving but when they get very worn you'll hear them clunking while going certain speeds, however if/when you hit bumps it would stop then come back again later. If the car was on the ground, and the CVs are worn then you should be able to feel play perpendicular to the shaft. Typically when the CVs need to be replaced it is because the rubber boot tore and let all of the oil which was lubricating the joint loose.

Well a vacuum leak is generally repaired with just a couple new rubber components, typically they are incredibly cheap to fix. When I say get a vacuum pump I'm refering to a hand pump like the Mity-Vac that you can pump the vacuum up by hand and then watch to see if it holds it on the vacuum gauge. It can be bought at nearly any auto parts store. Well from the sound of it your bad vacuum leak which is causing your door locks and vents to stop is what is causing your transmission to shift like it is. All of the vacuum comes from one source, however those two are on different orficies. Only so much vacuum can be created by the vehicle's pump, it appears as if your leaking to much of it out, you just have to find where.