Its very strange, at around 2000-2500 rpm, i hear a farting/rattling sound in the cabin. When i checked it ouit in the driveway, i found that it was inaudible from the exterior. Opened the trunk, had my brother rev it up and then i could hear it, farting away in my diesel tank!! I put my ear right up to the tank wall to make sure. Any ideas?

Well there are only three things happening in the trunk in neutral, the outside vent is trying to keep atmospheric pressure in the tank, fuel is being sucked through the fuel strainer by the IP, and fuel is being returned to the tank. If you pull the filler cap out and do the same what does it do? Does it get louder or go away? Are you feeling a lack of power when the tank nears empty?

__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

I pulled the filler cap off, no change. The sound didnt really get any louder or softer. I have about half a tank left, so itll be a few days until i see how it performs at low fuel levels. What can i check next?

Are you certain it is actually the tank doing it? Do you have the divider off? If not I'd pull that next then you'll be looking right at the tank, if it is more of a squeaking then perhaps the little mounts where the screws are could be bad.

__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

I have the divider off and I placed my ear on the tank itself. It sounds like it's coming from inside the tank.

Is the sender tight? I suppose it is always possible that the tube covering the sender came off and is vibrating in the tank.

__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k