valve problem?

I have a 1990 Ford AeroStar mini van. It has quite a few miles on it to say the least. I have recently noticed a tapping or knocking sound when the van runs. The sound increases in tempo when I drive faster. I poured a bottle of valve cleaner in and that helped very briefily, but the noise is back at full force. Could this be a valve problem? If so, what am I looking at as far as repair costs? And can this leave me dead somewhere soon?
Please help!!!!
Thanks in advance,
~Phil

A tapping valve could lead to a dropped lifter which spells TROUBLE...Pull the covers off and check to see if anythings loose or out of place....