Hello all, I'm hoping someone can help me troubleshoot a tapping sound on my 1969 220D Automatic. A few weeks back, it snapped the fan/water pump/alternator belt. So, rather than start the motor, I rolled down a hill in neutral. Yes it sounds goofy, but didn't want to heat the motor any.

Now, it has a tapping sound that's tied to the road speed of the car. (Not engine or transmission speed.) It's noticeable at 15+ MPH, and the frequency varies with road speed. If I'm moving along and I shift to neutral, the engine drops to idle but the tap stays exactly the same.

I'm wondering where this may be coming from. Did I kink the speedometer cable somehow when I came down the hill in neutral? I can't imagine I caused any internal issues to the tranny, any opinions? Shifting is perfect, engine runs like a top. I noticed the noise even before replacing the belt, so I don't think it has anything to do with that.

Any help will be much appreciated, I haven't been driving the car much since it happened!

-Rob

No, you should not have kinked anything simply rolling down a hill in neutral. Unless you hit something on the way down you aren't mentioning....

You say the noise is when you're moving. I wouldn't think engine...I'd be more inclined to look at something having to do with the actual movement, which would be wheels, suspension, etc. More info would help with the diagnosis. Exactly what type of noise is it? Does it change tone/pitch/frequency at different speeds? Is there any noticable shudder or pulling left or right? How about over bumps or around corners...does that make it worse/different? Have you gotten under it and had a look for anything broken or leaking?

There is a second oil pump in the transmission, so that you can jump start an engine or tow for short distances at low speed. I'm not sure if your 1969 220D has a second oil pump in the transmission though.

At what speed did you go down the hill?

Do I understand correctly that the tapping sound was there before the belt snapped? or only after it was driven down the hill?

Thanks for taking a look at my question! I'll try to clarify a bit.

I rolled down the hill at speeds under 10 mph. I'm in Florida, so it wasn't exactly a mountain or anything. I was trying to cover the 3 miles home without putting any undue stress on the engine considering the water pump wasn't turning. This meant driving for 2 or 3 minutes, and then waiting for 45 for it to cool with the hood open. A few cycles and I made it home.

The noise wasn't present before the belt problem, which makes me wonder if a shard of that belt is stuck somewhere and that's where the tap comes from. It doesn't sound metallic at all, and it doesn't vary with engine speed.

Thinking about it more, it seems proportional to wheel speed, and I'll pull the wheels this weekend to see if something may have flown in there when the belt gave up.

I've been under the car, and nothing obvious is broken. No new leaks since then, and the oil looks normal after that incident as well. My primary concern was not overheating the motor and damaging a head gasket or internal bearings.

The car runs, shifts, and drives just like it did before... which is to say the powertrain is great. No pulls or anything like that, and I haven't noticed any difference in corners.

Thanks again for helping me sort this out!

Got the car up on jackstands today, and can't see anything wrong under there. Everything looks just as it did before. The sound seems to be near my right foot, which is making me nervous.

Any ideas?

Right or left, front or back?

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1985 300D, 250k
1980 300SD, 180K

The car is US spec LHD, and the sound seems like it's near my right foot when driving. It's noticeable above 15-20 mph or so.

Can you hear it, feel it or both?
It's not a rock or some other object caught in the tire treads?

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1985 300D, 250k
1980 300SD, 180K

Hear it, I may pull the carpet and see if I can feel it.

I pulled all 4 wheels when the car was in the air today, and everything looked tip top...

It may be the steering damper. When you had it on stands, did you check for play in the steering? When you inspect the suspension, you have to look carefully at all the boots and bushings, moving things about...looking for movement or split books or worn bushings. It may even be something inside the car, like the shifter bushings...they may have crumbled away and now the linkage is clanking about. Looking at the undercarriage, look for clean wear marks too.

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IF the sound/sensation is rhythmic then wouldn't issue be associated with tire or bearing - something spinning?

OP stated while at speed and shifting to neutral the engine slowed down but the sound remained constant.

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1985 300D, 250k
1980 300SD, 180K

Yeah, whatever is going on is directly proportional to road speed. That's what had me wondering about the speedo cable. Under the car, everything in the driveline looks solid. Flex discs are newish, and nice and tight. Wheels spun freely and easily when it was in the air today, bearings seem good.

Anyone know how to disco the speedo cable? I don't see anything in the shop manual about it, and it wasn't immediately apparent when I was underneath. It still seems like a possible culprit to me just because it meets the road speed criterion and routes right near the area that seems to be noisy.

I don't suppose you have a rock stuck in one of your tires....

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2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

Can't find a picture of the connection of cable to transmission but is easy enough to locate. here are the instructions:



Found it

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1985 300D, 250k
1980 300SD, 180K

Pull it and have a look, by all means, but I have to be honest...I've never heard of anyone knocking their speedo cable out of whack simply by the series of events you've described. Have a pal stand next to the road and drive by...see if they can hear the noise outside the car. That'll help narrow it down.