[The Duke]

My 06, 38 open had same problem.   I found a rebuild kit for around $9.00 from Grainger.  if you’re going to take the caps off to clean it out, you might as well change them.  Really easy to fix.  If you go this route, remember the caps are plastic so don’t use a screw gun when replacing. Just snug them up by hand. I also ran a piece of stiff electrical wire down the air line from the roof while I had someone holding the horn button down. This will help make sure the tube is unobstructed and if there was some debri that got in there, the air flow will purge it.  I was unable to include the link but here’s what you should check ou from Grainger. 

FIAMM Horn Replacement Kit: Air, 3 in Lg, 3 in Wd, 1/2 in Ht, 3 in Dia.
Item 53CU34
Mfr. Model 21323