I have a C class 1.8L Kompressor 2004 mercedes sportcoupe- very like the w203.

There is a high-pitched sound coming from the rear of the vehicle. Equally loud inside and outside drivers’ door. Lower noise in front of car with & without hood open. Less noise on passenger side than driver’s side.

Initially this prompted me to replace the fuel pump. The old filter was cruddy and dark brown but it appeared to be working fine.

Even after this the sound persisted although its somewhat more consistent with the new pump. During hard acceleration the noise notably & reliably decreases. The noise can occasionally be heard when at a standstill. Depressurising the fuel line according to the workshop manual appears to fix the sound issue for a while. Occasionally when leaving the car sitting unused for medium periods of time ~1 hour the sound is not present for a while, considerably longer or shorter appear quite reliably come with the noise. The sound does not appear to be affected by speed. The sound does not appear to be affected by gear. The sound can be heard occasionally (somewhat rare) when turning ignition on without engine start – assumption is that this is due to the fuel pump priming process.

This prompted a check under the rear seat drivers side tank. Both side of the tank are clean. Siphon functionality appears function as at ¼ full tank of gas the drivers side tank was empty. The presence of the sound initially when the problem appeared to be worse at low fuel levels than high fuel levels. However now the sound persists at any fuel level.

My current suspicion for the cause of noise is fuel filter & some kind of flow restriction noise. The fuel filter is an aftermarket (mostly unmarked) 4-pronged filter, looks like it should according to the manual, and its relatively new.

Next work session will be to attempt to run the pump with the lines off the fuel filter into a standalone tank to attempt to find any kind of blockage in the line leading up to the fuel filter.

This is the first car I’m working on so even tips that may seem obvious that may help. I’m looking for help primarily for debugging tips.

  • Welcome to Motor Vehicle Maintenance & Repair! Commented Mar 11 at 22:50

1 Answer 1


Try Star Tron Diesel treatment at a 10x concentration. It's not a solvent and it's impossible to use too much. It might just give you the cleaning you need. I use it plenty in my E350 and love it. You can also combine it with some automatic transmission fluid in the gas without harming anything in the engine, at a treat rate of 4-8 oz. per 10 gallons of gas. It's an old mechanic's trick.

I'm not sure in which country you live. If it's the U.K., try this link.

  • Im in sweden. We have cleaning additives, however this is a gaoline car not a disel. So I would have to account for that. +1 for fuel additives though. Commented Mar 12 at 9:59

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