How to isloate turntable motor from making a hum

My turntable has an AC motor that is mounted right at the bottom of the turntable. The turntable itself is placed on top of the vacuum tube preamplifier. This placement makes the AC motor only half inch away from the pre-amp. Spinning at 33 amp produces a barely audible hum. But once it is switched to 45 RPM the hum level increases substantially. This morning I lifted the turntable up holding it about 10 inches above the pre-amp and the hum was gone! Once I lowered the turntable closer to the pre-amp the hum was gradually increasing reaching its max level once the turntable was placed right on top of it - when the distance between the motor and the pre-amp was shortest. Apparently, there is some form of interference going on between the turntable motor and the vacuum tube preamp.
Unfortunately, there is not enough space in my room to position the turntable to the side, next to the preamp. And I can’t raise the turntable 10 inches above it as it would make it hard to reach. I wonder if there is any isolation material I could use to place in-between the turntable and a preamp? Can an acrylic or plastic padding or a metal sheet used to block the interference?