2015 Nav with engine vibration

[Berrie]

So we purchased our 2015 Navigator L more than a year ago, with 70k miles.
Since then it has developed some vibration in the engine. It now has 97k miles.
It is not strong, and it is a rather fast vibration, like a drumroll (speed-wise, there is no sound to it).
The vibration is either not there or not noticeable when accelerating faster, and when going 75+mph on the highway.
It can be felt when the car is not moving, with the transmission either in P, R, N, or D, no difference.
It is at its worst between 30 and 45mph, particularly on a flat road; the car will switch up into 6th gear and run barely 1k rpm.
I have manually forced it into 5th and 4th and 3rd at that speed, and the rpm goes up but the vibration stays.
Has anybody experienced this, or know what it is?

 

[Action]

NVH (Noise Vibration Harshness)

The diagnosis for NVH is typically to eliminate things until one or just a couple things are left.
Since the vibration can be felt when the vehicle is NOT moving, that eliminates all of the suspension and much of the power train that isn't moving when the vehicle is stationary. Basically, what is left is the engine and a few pieces in the transmission.

Possibilities:
Drive belt (Or serpentine belt)
Things that are being driven by the belt
Engine/transmission mounts
Something that is bolted/mounted to the engine driveline and is loose. (Exhaust could be a possibility)
Something internal to the engine or transmission. Rather low possibility given the mileage.

Some diagnosis .
General inspection, especially under the vehicle, would be a step
Removing the drive belt and running the engine in park
Not sure if transmission fluid level can be checked, and that would be something to inspect.
A large/long metal pry bar to move the engine while vehicle is in park and the vibration is being felt.

For this situation .....................
"I have manually forced it into 5th and 4th and 3rd at that speed, and the rpm goes up but the vibration stays."
Does the vibration change in any way after the gear change?

Action

 

[Berrie]

@Action
"I have manually forced it into 5th and 4th and 3rd at that speed, and the rpm goes up but the vibration stays."
Does the vibration change in any way after the gear change?

I am unsure about this. I did try to get that feel, but the difference in engine rpm is not that much, and the vibration is very fast, so I couldn't say for certain whether the vibration changes with engine rpm or not.
I will check fluid levels, V-belt, and belt-driven items this weekend.