Colebob, the aspirator motor and the vacuum swithcing valve are both good suspects, as detailed by the guys above. But yours "might" be the speedometer, if it just happens during acceleration. Mine ticks just like the old cable units, even though it has no cable going to the tranny (magnetic pickup). I had it replaced while still under warranty; now the 2nd one is doing it after 15 years (acceptable I guess).

DG