I was afraid of the vibration on Manly's mounts, so I ground the edges off of the inserts to allow a little rubber flex before the polyurethane contacted the steel. On my bench grinder I took off about 1/8 of an inch to allow a little movement. I did this all the way around, and I have minimal vibration, almost not noticeable. As I did not install them without the modification I cannot tell how much of an improvement.
I am very pleased with these.