There really are not that many problems with a Diesel like the one you have that happen at one end of the rpm range and not at the other. If you have the hood open and start it, can you give some indication of where the noise is coming from and how loud it actually is? If the problem is serious you will be able to detect some signs of the problem, even at idle. From what you have described, if anyone could tell you what the problem was, they would be very wealthy as they could win lotteries every week. There is just not enough information.

Oil pressure dropping as engine conditions change is something to watch. Nearly all of these cars will have an oil temperature vs. pressure character you have to become familiar with in order to be comfortable. At idle the oil pressure can drop quite low, if the engine is hot. Once you apply throttle and the engine starts to take a load, the pressure should peg at "3" or "45" depending on how your gage is calibrated.

These engines are also pretty noisy, but the noise is very regular, and you get to the point where you actually enjoy listening - it is so regular it becomes reassuring to listen to it. There are things that can upset the sewing machine like regularity - and these are often things that fatigue due to vibration and heat. Like the little air cleaner mounts, or the EGR connection, or any of the belt driven auxiliaries on the front of the engine. And the valves or valve train/gear. The valves are driven by a chain that has a bunch of sprockets and driven loads besides the valves, as well as a chain tensioner and various guides. All this stuff makes noise and they are all susceptible to wear and when they stop working correctly they can make more noise.

So, after all that, you need to see if there is something seriously wrong and then narrow things down. Look into the radiator/reservoir and see if there is oil there, or look at the oil on the dipstick and inside the oil fill cap and see if there is any white goo. Oil in the wrong place or water in the wrong place is a sign of a head gasket failure, which could contribute to higher temperatures and a change in oil pressure performance.

If there are no signs of a head gasket failure, you will have to turn the engine back on and listen carfully to find out where the noise is coming from. Poke around looking for loose parts. If it sounds like it is inside the front of the engine, you may have to take the valve cover off and look in and around the chain, the guides and valve train. Also, check the throttle linkage out carefully. It is a bit of a Rube Goldberg arrangement, and can loose a rubber bushing here or there and let the engine idle go too low (stalling and low oil pressure), as well as make noises as parts now rattle against each other.

Until you find and correct the problem I would be somewhat hesitant to run the engine for any significant length of time. So, my suggestion is look around and report back. With more info, someone will be able to help. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)