These are unrelated, but I packaged them together to avoid cluttering the forum with my little questions.
Was doing my regular under-the-hood inspection and noticed a couple things. One is that when I loosened the resevoir tank cap, it made a hissing sound ... this was after the car was sitting for over 12 hours. I've done my searches like a good forumite, and I see that this can be the sound of normal vacuum. The up rad hose was indeed soft, BTW. I'm assuming it was vacuum, but in the past it has never made a noise when I remove the cap cold. What would cause a change, and how can I be sure it isn't pressure?

Also ... what is a good idle speed to set the car to when it's fall weather (50s, 60s, drops into 40s at night) and then for winter? Right now my indy left it at the "2" setting, and I feel that might be a bit low as it gets colder. Should I adjust it?

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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 inrush of air into the system is normal. Sometimes it develops a bit of a vacuum after it cools. The variation would be caused by the hottest temperature it attained during it's previous operation.

The only cause for concern is if it built up pressure in the system. If so, the upper hose will be very hard to squeeze prior to opening the cap. Then, it will be easy to squeeze. If it's vacuum, it's easy to squeeze in both scenarios.

Idle speed is somewhat of a personal choice. Some folks like the vehicle to remain at a standstill at idle in gear. Some want it to move forward. You don't need to adjust it for the winter as the computer will maintain the same speed despite the season. But, play around with it and see what you prefer........it's just a turn of the dial.......you won't harm anything.