98 windstar engine rpm

just did my 160 000 km service - plugs, wires, belt, etc. all seems ok except for something new i noticed - as the engine accelerates or decelerates (in park or drive) it produces a low pitched rumble at approx 900 rpm and goes away after passing that point. plugged in my code reader and nothing comes up. also, can anyone confirm what the engine rpm's should be at 100 km/h (3.8), i'm at 2000 rpm, seems to have gone up since the service. appreciate any help.

Thats to fast. Look for a vacuum leak. You might have a vacuum hose off or cracked.

On my 2001 its around 980 1012 rpms at about 30 sec. after start up..Or thats the reading I am getting on my scan gauge. Should be about the same on yours I would think..