79 300SD questions (newbie)

[John Winn]

Hello experts!
I am in my first week with my new old MB, 79 300SD and so far these are the things I need help with:
1. When I bought the car I had the mechanic do a compression test, passed with great compression. Had him replace the timing chain and adjust valves. Car starts on a dime and gives back change, but is REALLY slow off the line,expecially when cold..When warm it is boggy till the turbo kicks in then it goes pretty good..Little to no smoke out the back. Should I have had him check the pump timing too? Could it be retarded now? Also the vacuum thing on the injector pump buzzes when you turn the car off, like it is bleeding off.
2. The electric cooling fan in the front, should it run with the ac compressor on? It never comes on. What should I look for there?? Where are the relays, in my 240 they were in the fuse box but not in this one.
3. The cruise control works when it wants to. When it works it works perfectly, but sometimes won't engage at all..Whats the first thing other than fuse I should look at?
4. Lastly (for now) the car has a strange vibration above 45mph. It is more of a hum that you can hear than a vibration. Pitch increases with speed. I thought it was tires but had them replaced..It is not affected by throttle pressure, and doesn't go away when I shift into neutral...Any ideas??? Any help would be great, I haven't owned a MB in a long time so haven't gotten familiar yet. It is going to the local shop (Ben's workshop Austin TX) Wednesday to have an oil leak looked at..Thanks for any help everyone! John
PS, it has 285,000 miles on it.