I am looking at a 1999 Dodge ram3500 24valve 5.9cummins.. truck is in great shape and runs great.
Only thing im iffy about is after i test drove it and turned the truck off the turbo was making this clicking noise like every minute it would do it..the dude said it was the turbo cooling down but idk much diesels... is it the turbo going bad? Or what

The truck has 154000 miles on it and he wants 7500 for the truck..its a dually

 

Well, was the truck driven hard before being shut off? If the truck was shut off immediately after being run, it is highly possible the turbo is frying the oil inside. which could possibly be causing it to click or pop.
Turboed engines need to be let to cool down before shutting off, because of the oil suppling the bearings. When the engine shuts off, the oil stops, so if shut off hot, the oil in the bearings can be cooked so to speak. causing premature failure.

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Oh, is the truck 2wd or 4wd. 7500 seems a bit steep if its 2wd. I had a 99 3500 2wd back a bout 3 years ago. Mine was in about the same condition your describing. I had a reall hard time selling it for 8000k

 

truck needs a turbo timer.