My car has developed a loud metallic tick noise that happens every 30 seconds or so at idle. I would guess it might be the vacuum pump or belt tensioner? The car runs completely fine otherwise and the idle is smooth even with the tick noise present. I can't pin point where the noise is coming from so before I start taking things apart I figured i'd see if anyone has encountered this before.

IMG_3738 by jon allie, on Flickr

IMG_3739 by jon allie, on Flickr

Removal and inspection of the vacuum pump is not time wasted. On the 603 engines.

I had the vacuum pump off about 5,000 miles ago when I swapped the engine. It is the updated version and showed no signs of failure. Although your right it isn't time wasted and for all I know this noise could of been there for a long time. It's not noticeable unless you have the hood up engine running.

The long time between ticks interested me. A stethoscope type approach might help. Might not. I do not think your issue is particularly common. From your description.

Hey Jon,

Post a video of the sound, if you can..

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1983 300D Midnight Blue
4 speed, W115 intake mani, non-EGR exhaust mani, KKK K26, 3 pc Euro bumpers, Lo-spec Euro headlights, AL129X, hubcaps, custom A/C

1987 300TD Smoke Silver
Euro headlights, thermostat relocation, coolant bypass mod, rebuilt Becker + Jehnert + Helix audio, OEM Oris roof boxes and surfboard racks, Euro towbar

1983 300TD Reed (Moss) Green / Dark Olive MB Tex Euro spec

1983 300TD Silver Blue / Blue cloth Euro spec, OM648 and 722.6, 15" hubcaps, W126 S2 brakes

If you click on the pics posted they should take you to my flicker account to see the videos. I should of posted that to be more clear. I think it sounds like a sticky valve possibly? I believe it's coming from the valve cover and not the front of the engine.

For locating noises I like to use a section of hose about 3 feet long, something easy to handle, with a decent sized ID like heater hose or some 1/2" vinyl tubing from my scrap hose bin. One end goes up against an ear and the other end is used to chase down the sound you are after.

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- Greg -
1973 220D, The Prodigal Benz
1974 240D

In the three 603s I've owned I've noticed a bit of a lifter noise at idle as they've aged. It comes and goes. Maybe/probably they're not getting consistent oil at idle; not sure. The second I bump the rpm to above 800 or 900 away the noise goes. I cannot see how if you have the updated vacuum pump that it could make noise they seem well built to me. I would look at the belt tensioner and spring and shock. That is a real weak point on the 603 and needs changed out more frequently than you'd think especially with some of the aftermarket crap that's out there.
I do advocate the stethoscope method. They are cheap and surprisingly will let you hear very specific areas of the running engine.

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DS
2010 CL550 - Heaven help me but it's beautiful
87 300D a labor of love
11 GLK 350 So far, so good
08 E350 4matic, Love it.
99 E320 too rusted, sold
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www.Newgateschool.org - check it out.
12 Ford Escape, sold, forgotten
87 300D, sold, what a mistake
06 Passat 2.0T, PITA, sold

Las Vegas NV

It's the lifters.

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CENSORED due to not family friendly words

Lifters can internally varnish until they stick. Either synthetic oil can help or a quart of transmission fluid in the base oil.

With lifters other than the ones that are dirt cheap. I number when removing them. Then pump them up with a drill press in tray a of light oil. Seeing how they compare to a new one.

If not I take them apart and clean them. Then retest. If they then test as good as new there is no wear.

I am not suggesting this is your problem.

Thanks for all the responses guys. The noise has come and gone thankfully. I hadn't driven the car for a week so a lifter probably had bleed down. At idle the hot oil pressure is very good at 2 bar. My top end was rebuilt 10k ago, along with a low miles bottom end so I should be good to go for quite some years to come!