Hello dear forum!

300TD with SLS and ASD, manual, no turbo,ZHM oil in the system, oil is 4 years old, level is at minimum.
I took the 300TD to travel 900kms from home and since a few days, i have a tonk tonk under the rear right seat.
The noise is not super loud and it is there as soon as i start the engine, the rythm is a tonk per second and it doesn't change with the engine revolutions.
I isolated the noise coming from the ASD module under the car. One of the hard lines is pulsating at the same rythm as my noise.
I tried removing the ASD relay under the hood without success, disconecting the ASD module didn't help either...
The car is running fine, but this noise is worrying, i am far from home and don't want to drive it like this...

Any feedback greatly appreciated.