cub cadet xt1 lt42b brigs and Stratton engine struggles turning over and struggles starting

[randarrington]

I've got a 3 year old lawn tractor and by the second year I noticed that it started to struggle turning the engine on the start. Once it had started then there wasn't an issue. Now, however, it seems to have gotten worse. The motor will turn once and then struggles to turn again. Holding down on the key will eventually (until the battery dies) turn it again but it's not fast nor consistent enough to fire the engine. I thought it was the battery since it seems to drop below 10 v when I turn the key. So the check this, I tried jumping from my car batter which produced the exact same performance. Sometimes I'll hear a high pitch whine as it struggles to turn the engine. I'm guessing it's either the solenoid or a bad starter. I did hear that the belts on the deck could be tightening and causing additional resistance. I'm not sure how to even check that. Does anyone have any advice please?