I felt compelled to reply to this thread since I always joke about it with fellow students. Myself and others tend to think that saying ahhh is just a trick used by those professional types so that they seem calm, collected and cool on the radio whilst conveniently hiding the fact that they are actually struggling to remember what the hell to say. I always noticed that where as a student pilot sounds like an idiot on the radio with long pauses and ummmmmm's the Q Link blokes sound like pros with their ahhhh's or more like uhhhhhh's.
That's my 2 cents. I've started doing it myself now when I get stuck on the radio, I recommend it to everyone.