Two unrelated questions...
- How fast should the oil pressure indicator engage after the engine is coldstarted? I was just curious, because I'd noticed that right after I start my car, there is a short pause, then the gage shoots right up to the top ... is that what it's supposed to do?

-Tonight when I was moving my car to a different parking spot, I noticed the transmission making a sort of "hissing" sound when put in reverse. It sounded almost like a "pressure release" sound if that makes sense as a descriptor. It would last for a few seconds after the car was put in reverse, then disappear. Any ideas what this might be? I hate to think ...

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

The delay in oil pressure is normal. It's just the amount of time required for the oil to get from the pump to the sending unit at the top of the engine.

Can't comment on the transmission noise, however.

A 'somewhat' related question:

At what oil temp does the oil start flowing thru the cooler? The '82 300SD has a pegged oil pressure guage. The other day I took it up the road a couple of miles, water temp was above 90c, let her set and listen to the beautiful sound of that shakey 617 for a few minutes and then touched the oil cooler line. Cold! Well, much cooler than every thing else under there.
I felt both lines up to the oil filter tower and I gather it was NOT flowing.
Could this be related to the pegged guage?

Can't recall the exact temperature..............seem to remember about 95°C. or so.

But, I can tell you that driving it up the road for a few miles certainly won't get the oil hot enough to flow to the cooler. You need a decent run for this.

Maybe it was just under the oil t-stat opening temp. I'll check again but,
It was actually a few miles. Just used the word 'couple'. It could have been five to ten. The car is work in progress and she is currenty not registered. So, I stay on the rural paved roads around the ranch. Some straight passes at near WOT.
It was certainly toasty under the hood.
One thing I did notice was how much larger the aux fan is on the gas (380se) car.

Thanks.

My 300SD does this too sometimes (though it is VERY quiet and VERY hard to hear). It sounds to me like a fluid build-up/release inside the transmission. If everything works dandy as far as gears go then a leaky seal or noisy pressure fill/drain really isn't an issue other than being noisy. If it is louder it could be a larger issue however I am not sure the level of volume we are talking here

Imagine filling a given space with hydraulic fluid...that fit the sound?

If it is as quiet as mine...you've got a good ear for the drivetrain noises

EDIT: Forgot to mention...my transmission works flawlessly and makes this noise on occasion...so no worries

According to the 617.95 engine manual, the thermostat begins to open at 110 degrees C and fully opens by 125 degrees.
Except, on certain W116 models, the numbers are 95 & 110.
Keep in mind that those figures are oil temperature, not coolant temp.