Maybe a bad ground. My dad had a pickup where it lit up with all kinds of problems. Turned out to be a corroded ground connection.
I had a car that popped up all kind of warning stuff and it came out to be an alternator beginning to fail.
Kind of hard to figure out when it is just described as noise. What kind of noise is it making, roar, squeal, knocking?
What do the belts look like?
Have you checked the blades for damage?
Do the spindles have any play?
I would think the old Briggs engine should not be a problem. I bet the deck will be your headace. What kind of drive unit is in it, some type of transaxle? Really does not look too bad considering the age.
A lot of people apparently can't pay for their $80,000 SUV/Pickup any other way. Low or no down payment and finance the whole thing. Chances are good they will trade it off before it is payed for and repeat the cycle again.
Call me crazy, but I have converted both my zero turn mower and my 12000 watt generator to Amsoil small engine oil. Mower is 15 years old (approximately 600 hours) and does not use any oil. Generator only has about 30 hours mostly test runs uses no oil.
Had a subsistance farm uncle who used it to grease his boots and other leather items. When he came to our house our cat and dog would follow him around. They were looking for a bacon handout.
The failure mode on those motors is the head gasket or the cam mounted compression release. I do not think it makes any difference what oil you use just so you change it regularly. You will probably get about 400 hours when the head gasket will completely blow or it will suddenly start sipping...