john deere stx38 safety switch now noise?

Hello everyone...as usual, working on something for a neighbor. This is a John Deere STX 38 model M00STXJ299848 . It will start and throttle fine but he had to 'jump' or 'tie down' the brake switch to be able to drive it because as soon as he let off the brake it would die. He drove it out of the garage, engaged the PTO and it died. He did this a couple times with the same results. I checked the engine ground there by the right front wheel and it is secure. I checked the PTO switch, seat switch and ignition switch using his technical manual (I rarely have access to something like that!!) and all checked fine. There are no obvious scrapes or breaks in the wiring.
I read in another forum about clipping the white wire at the PTO switch to test something but I didn't get that far......
I started the mower, engaged the PTO and it started making a sharp metallic non rythmic knocking sound at the pto clutch. I shut it off, scratched my head then started it again. It now makes that knocking even without the pto switch on (it still dies as soon as you engage the switch).
I guess I now have two questions.....first - any ideas on the knocking before something messes up? and second - any ideas on this electrical/safety switch problem?

 

The electrical problem really sounds like the seat switch. I'm pretty certain of it.

The metallic sound is probably bad bearings in the pto clutch.

 

thanks cheese,
sorry its taken so long to reply and i wish i had results but the owner fell under the weather so maybe we can tackle it next week. The seat switch makes sense to me but he seems to think it is the starter solenoid because that is the only thing he replaced...i know, that makes no sense to me but my question to everyone is - is there a way to isolate the seat switch from the equation so he will see that before he goes out to buy yet another starter solenoid? I know I can't jump it. I'm guessing we could remove the black wire from the coil to disable all safety and choke it to kill it?

as for the pto bearing.....makes perfect sense! i'm abit rusty :-)

 

The solenoid can't be the problem. It doesn't have any way to kill the engine. The solenoid only is operational during starter operation and then it is un-used until next time the starter is engaged.