How many hours on the machine?
Lifetime lubricated bearings are generally misunderstood. People hear that used in the product advertising and they think "I will never have to lubricate those pulleys". Correct in one sense but it really means that once these start to fail for whatever reason their lifetime is over.
MTD has a lightweight/low cost approach to certain things which varies depending on what model series you have. I have a Cub 2518 that I have had for approx. 10 years now and I am now pushing 500 hours. Biggest repair frequency I have faced has been the rotating stuff on the snow blower attachment (Michigan with a 1/4 mile long driveway).
Have you replaced the mower deck spindle and idler bearings yet?