Problem ? May be maybe not ?

[Beastie]

Hi all, Here we go. 85 300SD have been tinkering with it and trying to add some miles and wring things out some. I know that a valve adj/check is in order as well as installing new glow plugs but a new issue has come up. I took the car for a drive on the highway and ran at 65-70 as all the traffic was doing. I kicked it down and it ran upto 80-85 and I slowly allowed it to decelerate again. No problems it actually ran smoothly in chassis and engine. I came home and parked it overnight and drove it again and noticed at speeds between 30-45
there is a fine vibration coming through the accelerator pedal. It disappears almost completely over 45-50. It is an extremely fine vibration and goes up and down with the engine acceleration so it is not drive line. I replaced the belts
2 months ago and they seem fine I may check the adjustment again. It is almost like a humming vibration similar to an electric motor. I am going to check and see if the exhaust might be rubbing or have a loose point somewhere but I do not feel that is it. The tranny seems to shift fine and firm. I just got back from a short highway drive and I believe the power level appears down almost as if the turbo is not spooling up or kicking in at lower speeds the way it did before. Any thoughts ? I know this is vague but I will not be able to take a closer look for a couple of days.
Thanks
Alan