The injection pump on your car is more or less just an electronically actuated mechanical injection pump. It's not really sophisticated enough to trim fuel like a gas car or the later CDI type engines with full O2 feedback control loops. At 5000 feet elevation, there's considerably less air for the N/A engine to work with, you'll always be running a little rich compared to running at sea level.

The noise you're hearing could be injector related. Considering that you had the injectors rebuilt, it would be worth checking pop pressure and spray pattern. You could have an injector that's dribbling or spraying funky. Another possibility is EGR function. The altitude capsule on the 603 based cars is to help tune the EGR operation more than anything else, and I'd expect it's not much different on the 606. Given that EGR dilutes the fresh air charge with exhaust gasses, you can expect a bit more "clattery" noises, especially since you're already down on power from the lack of air due to altitude.

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Current stable:
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1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

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