Each blade relative to it's spindle should not have any play at all - tight in all directions. Closely recheck blade mounting, cleaning matng surfaces well first.

Bad vibes can certainly come from a belt that the sides have burn spots in them too. You didn't say why you had changed the blades , but seem to indicate that the problem is new? It really wouldn't be a bad idea to have the blade balance checked on a HOP blade. New, unfortunately doesn't mean sharpened on both sides let alone balanced...

Concur with Tom, when I put mulching blades on my GT found two balanced and one badly out of balance "right out of the box." Pick up a plastic blade balancer, they're cheap and use whenever you sharpen, change or do anything to the blades.

Good luck,

Ev

Something to check when you have the blades off of the deck:
Stack all 3 blades on top of each other so the ends are lined up.
Stand the stack on end and "sight through" the center holes.
The blade center holes should be almost completely in alignment with each other.
Flip the stack over so the opposite ends of blades are pointing up and recheck the center holes alignment.
The center holes should remain in the same perspective as the other way.
If you see that one blade center hole stands taller (than the other 2) when stack is on one end, but stands lower when stack is flipped...........the center hole on that blade was not put where it belonged when the blade was made.
I have seen this just once in my life. It was discovered by a friend of mine after he replaced his blades. One of his blades had the center hole 1/8" off sides. He was able to get it swapped by the place where he bought the blades when he took the blades back and "stacked them up" on the parts counter.