Post the long model # from the tag on the rear of the deck. It sounds as tho fro, the noise that either the transmission is jumping hard enough to hit the housin g or it is getting bound up inside- which is why I'm sure you serviced it. It may be time...

QUICK AND EASY!! Tools Needed: Not much -- one foot length of 1/2' nylon rope, vise grips, and large ratchet wrench with 5/8" socket Now you can spend a lot of time trying to figure this out on your own -- there are a lot of ways to skin this cat -- But do it this way and you'll be done in 10-15 minutes with a little practice. 1. Loosen deck deck belt so that it can be removed from PTO pulley by "rolling" the belt off the PTO pulley (or spindle pulley). From rear of mower reach underneath with both hands and pull belt with left hand downward and rearward while turning PTO pulley to right. Maintaining downward tension while turning PTO pulley edge belt off the pulley. 2. Remove old transmission drive belt. Typically will already be broken or hopelessly worn so from rear of mower, section with knife and discard. 3. Lower PTO assembly from rear of mower to thread new transmission belt onto engine pulley. NOTE: This is necessary as there is an anti-rotational bracket that is welded to the framework and prevents the belt from being positioned in place otherwise. To lower the PTO assembly you must loosen the 5/8" bolt at the bottom -- but to prevent the flywheel from rotating while loosening the bolt you'll first need to remove the spark plug and feed into the spark plug hole a foot or so of 1/4" nylon rope. The piston will come up, hit the rope and lock up the engine. This will allow you loosen the bolt at the bottom of the PTO assembly. And the nylon rope will not hurt the piston or head -- just remember to leave enough rope out to remove the rope afterwards. Once the PTO assembly bolt is loosened slowly unscrew ONLY ENOUGH so that the PTO assembly drops below the anti-rotational bracket and the belt can be put into place. Use the notch where the anti-rotational bracket engages the PTO assembly to facilitate placement of the belt above the engine pulley. Once belt is in place retighten the PTO assembly bolt making sure the anti-rotational bracket is aligned with the notch on the PTO assembly. Remove nylon rope from spark plug hole and replace spark plug. 4. Finish placement of belt. To complete placement of belt you have to remove tension on the transmission idler arm. So begin by detaching the spring from the mower frame to release the tension on the idler pulley so that the belt can be placed. Read below on methods to replace spring. From the rear of mower FIRST thread belt around the right transmission drive pulley and then onto the left transmission drive pulley. The transmission drive belt is then positioned onto the engine pulley at the top of the PTO assembly and then on the drive idler pulley. Tension is then reapplied by reattaching the tension spring. NOTE: Replacement of the tension spring is a "simple yet a hard to master" move. Using a pair of vise grips placed right at the base of the spring's hook and oriented so the hook is at 90 degrees to both the hook and the hook's latch on the framework the spring is pulled into position and hooked onto the latch. Adjustment and placement of the vise grips is critical to prevent near misses and frustration. And unless you have the upper torso of a gorilla, position your body so that you are using your legs and not your arms to pull back the spring. For those less inclined to use their legs and biceps you will appreciate the use of a wire cable or ratchet drive to unload tension from the spindle arm! The manual describes the use of 3/8" drive long-arm ratchet being placed in the square hole of the drive idler bracket and used as leverage for releasing tension on idler pulley; others have left the spring in place and pulled the idler arm back by threading a wire cable into the square hole in the idler arm and extending the wire to the underside of the framework to the right side of the mower where it could be attached to fulcrum and secured. 5. Replace deck drive belt. First place the deck drive belt on all mower blade pulleys and spindle pulleys. Then sitting at rear of mower looking forward reaching underneath grasping the belt with both hands pulling back and sliding the belt back onto the PTO pulley. 6. Remove rope from spark plug hole and replace spark plug. Job done.

Belt tension isn't adjustable. I'm suprised that these aren't spec'd as kevlar belts. I would check the Cub site when comparing specifics between models. Here is a link that might be useful: cub online

The left tension pulley on my LT 1045 Cub Cadet was about 1/2 inch to low and at an angle that took it out of a horizonal plane with the ajacent pullys. Took deck off and removed the bracket that holds both tension pullys. Put the defective pulley in a metal vise and hammered it so that its angle when reinstalled was parralell to the ajacent pullys. This solved the problem. I called Cub Cadet and spoke with the technical representative and he told me that the bracket I had to bend was bent out of shape because I, as many people do, engaged the electric pto to the blades while the deck was in level 3 to mow. He told me to put the deck in its highest position before engaging the pto to engage the blades. This alleviates the stress on the Kevlar belt that is trying to come up to 3600 rpm while in the grass before they start to rotate. This can eventually bend the bracket that holds the tension pulleys. He said he is trying to tell the company to include this in the owners manual.