Shakes in Serpentine Belt system?

Yep, this happens when the serpentine belt tensioner goes bad, here is how to replace it, also you should install a new belt as well. Remove these three bolts after the belt is removed. While the belt is off, spin each accessory to make sure they spin freely. Check out the images (below). Please let us know what happens.

Understood. Could the alternator freewheel pulley be bad also? I’m going to a mechanic tomorrow and don’t want to change all the parts. I’ve just done the phase kit a couple of days ago along with water pump.

I don't think the one-way clutch can do this, but you never know. I would do the tensioner first; it is a common failure.