We recently had a new Trane variable speed heat pump/air handler installed. The air handler, which is in our attic, makes a low rumbling noise (somewhat akin to a flame needing oxygen) that the old air handler did not make, particularly when it operates in a higher speed mode. It also made a whistling sound until I put some painters tape around the edges of the air filter.

It is a 2.5 ton air handler and the air intake duct is 12" x 24". I've tried removing the grate and filter and the rumbling continues. Could the air intake be too small? Any other ideas for the source of the noise and possible solutions to try to fix it?

Thanks so much for any advice.