[rockadanny]

This is inside a CD player so transformer is small.

[FullRangeMan]

Rubber soft or mid density foots IMO.
As an audiophile alternative there is sorbothane.

[Speedskater]

You might be able to find some shoe inserts that are made of Sorbothane®.

[newzooreview]

Herbies makes a variety of vibration control feet, including some small ones:

https://herbiesaudiolab.com/collections/component-isolation

[Speedskater]

From Edmund Optics
https://www.edmundoptics.com/search/?criteria=Sorbothane&Tab=Products#

[rockadanny]

Thank you all and thank you for the links

[rollo]

   If you are handy try making a silicone ring. Remove transformer and put under it. Learned that from Angela/Gilbert Yeung formerly Blue Circle. You can also try Michael greens suggestion of loosening the transformer screws. Try that first.

charles

[qdrone]

VPI Magic Brick placed over the transformer on the outer chassis.

[FullRangeMan]

RF-Parts usually have rubber gromets that may fit this use.

[Tam Lin]

How about these?
https://www.keyelco.com/userAssets/file/M65p133.pdf
I have a few left over from a long ago project. They are similar to Keystone #773. If you PM me your address I'll mail you some. How many do you need?

[rockadanny]

Thank you! I found a small thin piece of sorbothane and slid it under.  Improvement!
Thank you all!

[andyr]

I use a pad of 1/4" (6mm) Sorbothane under each power transformer that I need to use in a build.

[John Tulett]

Pleased a successful result has been achieved, just to share my experience isolating transformer vibration / hum noise in my vintage Acoustat electrostatic speakers.
While I achieved some success with grommet mounting and anti-vibration sheeting, by far the most effective solution was found by replacing the bias transformer mounting hardware with anti-vibration threaded studs.

Please see, "Acoustat Interface MK-121-2 - Minimizing the Bias Transformer Hum Noise".
https://www.diyaudio.com/community/threads/acoustat-interface-mk-121-2-minimizing-the-bias-transformer-hum-noise.412612

With the Acoustat interface the bias transformer is mounted vertically to the chassis side, nevertheless, hope this solution may be helpful.
Thank you.