i have a reach in freezer with a tecumsi semi hermetic pumping 408-a. a several months ago i installed a valve plate kit, new liguid line dryer and sight glass. the unit runs and cools beautifuly. recently unit started with a knocking noise on start up and gradually goes away. i have not been called back on this, i just noticed it the other day while working on a adjacent cooler. when i left it last amp draw, superheat, sightglass, all good. condensor is clean. any ideas of posible causes?