Pearl Trillium diverter and hand held shower

That sounds like 'Water Hammer', cavitation is something completely different. You can put a water hammer arrestor on the system somewhere.

https://www.google.com/search?q=cavitation

https://www.google.com/search?q=water+hammer+arrestor

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Found your breadman TR500 paddle at the following site
Hope this helps
http://www.bonanza.com/listings/Kneading-Blade-Breadman-TR500-Trillium-Bread-Maker-Machine-Part-TR-500/140971587

Common issues...what is the problem?
Can you be more specific?

rod
us parts center

Do you have water coming out of the tub spout and not the shower head? If so how does the water get directed to the shower head,is it by pulling up the diverter on the tub spout?If water is still coming out of the tub spout then the diverter is bad and the tub spout needs to be replaced.

Assuming that your two heads are fed from the typical curved wall mounted shower pipe, what you need is a diverter valve. Removed both heads till it's down to just that curved pipe. Get a diverter valve from a plumbing supply store. Tell the sales folks that you want to install a diverter valve so that in one position, the hand held receives water and in the other, it cuts the hand held and puts it to the reg. shower head instead. They should give you a "T" sort of fitting. The female threads of this screws into that curved shower pipe. Then you will find two male thread ends. These connect to both your shower heads. Finally there will be a push/pull knob that is operated to control which head gets the water.
Nimsgame

No, you need to cut a hole. Usually an access panel is provided during the initial installation, either in the ceiling from downstairs or in the other side of the adjacent wall. But it's pretty easy to add one. After you cut a nice rectangular hole, you can cover it with a piece of painted plywood that has a trim piece attached to the plywood. The plywood is held in place with a couple of screws. When you remove the panel, the trim stays attached to the plywood. When you place the panel in the hole, the trim covers the gap between the drywall and the plywood.

This unit will allow the function of the shower head or the hand held shower individually. You can make both operate at the same time but the performance will be dismall at best.

you may have something in the line,take it off and check both the line and the handheld.

the connections are reversed,just change them over and this will fix the problem.