this only happens when im on the highway, and i freaked out thinking that it was an engine problem, but it went away when i turned the heat/defroster off. it makes kind of a very low, quiet, "wub wub wub" noise. if i turn it off and turn it back on again, it stops. whats going on with it?

 

if it adds anything, my dad thinks it might be a leaf stuck somewhere in the system, anyone know what it might be? thanks in advance