ooops ok the SM 420s are usually a little noisy after they get old and worn i suspect what your hearing is more like a whining noise which is common to these type trannies after several thousand miles my 63 GMC gets so loud you cant hear the radio if you really strain it lie pulling a 7% grade with 2 1/2 rick of fire wood in the back.if you dont see any loose debris or metal shavings i would just flush it out with kerosene and refill with lube.
