Of my 3 cars my 87 300D is the only that I've taken down to the lower elevation of central California and the Bay Area and I immediately noticed how much quieter the car idled over there compared to over here at 5000 ft where it has pretty loud diesel clatter. I've done just about everything on this car: inspected timing chain (2 degress stretched), injection timing, injectors, replaced delivery valves, air filter, fuel filters, ran Diesel Purge and using Mobil 1 synthetic. I'm starting to think that I have low compression, but maybe this behavior is normal as the thinner air at higher elevations effectively reduces compression. I'd like to hear from others who have taken their diesels to the moutains and back and listened to the idle.

__________________
2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

I have noticed this same thing when driving down to the Bay Area with my 240 then my 81SD and a couple of months back with the 84SD. I was kind of thinking maybe the air fuel mixture (ALDA) is a little out of adjustment for the higher elevations.
I am only at 3100 feet so the adjustments would probably be pretty minor.

I will probably mess with the 84SD this weekend as I have a bunch of maintenece to do. I'll let you know if It makes a difference.

__________________
'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique

I've read a little bit about the ALDA, but from my understanding of it it can only affect acceleration and amount of smoke, not how the engine sounds, though I'm not sure about that. I hardly get any smoke from my exhaust, except for a few seconds after a cold start, but I do get a pretty loud but consistent diesel knock at idle across all cylinders. Above 1500 rpm or so it's as smooth and quiet as a gas engine even at this high elevation.

__________________
2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

The only difference I can identify is the thicker air at sea level as opposed to to my elevation at home. The alda actually adjusts the rack ever so slightly to allow more or less fuel for any given setting.
Then it adjusts automatically for the altitude driven based on the air pressure so my assumption is that it could be adjusted to balance a little closer to my elevation. Of course then at sea level the car may get too much fuel but the adjustment it makes is so minor I doubt you would notice the difference.

__________________
'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique