Your ears are a very important limb in a milking parlour, as the lads were saying above. Also, your cell count and cow milk out are signs. A regular machine test is advisable, at least once a year. When I came home I was milking in a 6 unit herringbone with one vacuum controlled pulsator doing all 24 cups, it ran slower in cold weather and on really cold days a tap of a hammer was necessary to get it started... I won't tell you the cell count .
My Grandfather was one of the first around these parts with a dedicated milking machine, pulsation was started by giving the pulsation pendulum a swing, milk lines were made of glass as stainless steel was so expensive.