Plumbing – Vertical direction of PVC tee used for vent only

Couple options come to mind:

1. Change the fan.
2. Change the fan speed.
3. Change the fan connection to the vent.
4. Dampen the vent.
5. Change the size or length of the vent.

If you can't or don't want to change the fan, then try dampening the vent by clamping it somewhere to the building structure and possibly adding some padding or pipe insulation where it makes those connections. This changes the resonance length without changing the actual length of the pipe.

If dampening the vent doesn't work, then you can change the length of the vent. You mention that you can't make it shorter, but perhaps you can make it longer, or larger. Check the manufacturers requirements on the maximum length, number of elbows, minimum size, and required slope before making any changes like this.

Find a better store, if the ones you are going to don't have 2" PVC in full sticks (or possibly look higher up in the store you are in, and get someone to drag it down.)

In MY house, the only place you'll find 1-1/2 or 1-1/4" is the tailpieces on the sinks - as soon as it hits pipe, it goes up to 2", because it's not that much more expensive and it greatly reduces the odds of having any issue with it. I don't recall having any trouble finding it in full sticks. The main line is 4" (not 3") for the same reason. Builders looking to squeeze maximum profit don't do that sort of thing, but builders who plan to live in the building for a while sometimes do.

In retrofitting, you might need to use 1-1/2" due to space constraints. In any case, if you are redoing the pipes, you should probably bring the venting up to current code while you are at it.