For some reason, some Holley or Summit pumps will be noisier than others. I had one in my Jeep that sounded like you describe..........people riding with me would ask " what is that compressor running?" At redlights I would see other drivers looking to see where the noise was coming from. Yet I have the same model in my '27 and in my current T, and they are much quieter. In fact, I have to listen carefully to make sure my electric pump is running before I fire up the engine in my '23. My Son has a Summit pump in his T and it is also quiet. Don't know why some are worse than others, but it happens.

Rubber mounting does help, but the one in my Jeep had two thicknesses of rubber pads under it and the thing still rattled away.

Don