When I turn on the shower a whistling sound starts. When the diverter is pushed down so that the water goes in the tub, the noise stops. The diverter does not shut off all the water coming out of the tub diverter when it is in the shower position.

Is diverter on the wall or on the spout?

I have the same problem in my shower and my diverter is on the wall.

Pready

I suggest that you have some debris in diverter or a vibrating washer in the diverter.
Remove diverter, turn on water and flush diverter port. Check washer to insure that it is tight. Reinstall diverter.

Thanks for info Hkstroud, will do as you suggested. The noise is not as bad or a long as it used to be, so I think the diverter is dirty and your suggestion does make sense.