air dryer

I have a 1978 s series with a 290 cummins . The dryer behind the drivers side door constantly cycles making a hammering sound. Is this normal?

I'd say it's normal,if the humidity and temperature is high. If it's constantly cycling on cool,dry days,then i'd have it looked at. I used to drive a '82 S-2200 tandem,with DT 466,air brakes,and NO air dryer. I'd have to drain the water from the system every half-hour,or so,on 90-plus degree days! I WISHED that thing had an air dryer! I worked for an ice company,and holidays were our busiest times! On the Fourth of July,1994,I took the S-2500 tractor,with a Great Dane 48' reefer,since demand increased,and I had a larger route to cover. The "kid" that was used to his assigned truck,(with juice brakes),was assigned the S-2200. I told him to drain the system,after every stop,but he didn't heed my warning. He descended a mile-long 5% grade,and got going so fast,he couldn't down-shift the 10-speed "Ranger",and he rolled the truck at the bottom of the hill,even after applying the park brake! He said he heard it "Whoosh",but it didn't have any effect!

NO! it is not normal, nor correct. A properly functioning air system, when traveling down the hiway (no application of brakes, useing air) should cycle between every 10 min to 3/4's of an hr. Less then that you have a problem. I used to get 3/4hr out of my 9670 with air ride towing an air ride trailer.
Do you know what air drier it has? AN AD-9 is famous for the drier cartage comeing loose inside the dome, and will "hammer" when it blows off.
You need to find the air leaks and then tackle the air drier itself.
In a normal system, the air builds until cut-off pressure is reached, then the air gov activates the unloader on the compressor, and the same line also puts the air drier in to regen, by opening the purge valve in the bottom of the drier, this is the burst of air you hear every time the compressor cycles off.
In town, stop and go, the air system will cycle more often as air is used for the brakes, but it still shouldn't cycle repeatedly.
How often the dryer cycles has nothing to do with how humid it is, it cycles every time the pressure builds to cut-off pressure. once the system pressure drops to cut-in pressure, most often 15-20 psi below cut-out pressure, the compressor starts building pressure again, and when it reaches cut-out pressure the drier cycle. If it is cycleing in a short time period, it means that air is leaking somewhere, even if it isn't showing on the gauges. The Gauges only show the pri and sec air pressure tanks, but not the wet tank pressure.