Every time my Diesel climbs up a long hill the engine makes a sound like a variable wave: when I am the bottom of the hill and the rpm is high (~3000) it sounds like WoWoWo...and then at the middle of the hill the sounds wave amplitude decreases like WoWooWoooWoooooo and at some point there is no waive anymore (at about 2400rpm) and then while rpm continues to go down the waive is coming back : ooooooWooooooWoooWooWoWoWo..
The car is a MB190D with a 2.5 engine.(year 88, 160miles)
I asked a 'diesel' friend if a valve adjustment will fix this and he answered that this engine has hydraulic lifters and there is nothing to adjust there.
I run a diesel purge through the engine but did not help the wavy noise. I checked the exhaust for holes but I haven't find any.
Any ideas what this could be? Does anyone experienced similar noises ?
Thanks a bunch !