'wavy' engine sound while climbing up the hill
 

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 !

Probably a worn universal joint in the driveshaft. My wagon had a similar heterodyning noise under load before I replaced the driveshaft. The old u-joint had a lot of play in the center of it's range of motion.

Do a search in here on driveshaft vibration or driveline vibration...lots of good stuff in here on that.

Good luck, let us know how it turns out too, OK?

I'll second that, I was thinking the same thing. Check the center support bearing, flex plates, trans mount, diff mount, motor mounts too. If your car has them.

Thanks, I will check the driveshaft
 

Thanks a lot ! I will check all these next weekend.
:)

This is due to the Doppler Effect of the wavy hills
 

and the sound waves changing in frequency with your cars exhaust as it mirrors the waves in the hills.. Suggest you try some other hills, why not try some sharp choppy hills, and why not drive on some level roads and also cliffs.

Don't forget about the space aliens. Could be messing with his brainwaves via 1.21 gigawatt reverse-phase modulator.

Surprised nobody said tires..........

I had 2 sets of bridgestones that used to do that.

the problem is solved
 

Yes, the sound is now nice and clean; the universal joint in the driveshaft was replaced with a new one. (it was having a big play)
I haven't tried it yet on some sharp hills still... :)
Thank you all !