The problem you're describing sounds like the bearings. Where the crank is, there are bearings and a flywheel. When those bearings start to go out, there will usually either be sounds or a grinding feeling when you pedal. After some time it will get to where it becomes hard to pedal.
You will need to replace the bearings on the crank assembly to fix this problem. If you've not ever done that sort of thing on bicycle or exercise bike, it's not something I recommend you try to do. It's not a super hard job, but it is one where you kind of need to know what you're doing so you don't have to keep taking it back apart.
If you're up to try it, you can go
with your model number and look at the parts pictures. You can also order the bearing sets (takes 2) at the same site, part number 176991.
If you would rather have a tech come out and take care of it, you can arrange for that here:
Sears Home Services
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