My alibi is that it has been 28 years since my last AAR. But I could feel the connection most of the time.
Why? A little background. The airplane’s crew chief would periodically lube the receiver hardware (don’t recall the frequency). However it was not uncommon for the receiver hardware to fail to latch on to the boom. If that happened the boomer would send you back to pre-contact and have you cycle your IFR door. If the problem persisted, the boomer might “stab” the boom into the receptacle to get it to latch. So ISTR some boomers’ technique to smoothly stab you on their first try. I don’t think it’s my imagination that I heard a very quiet “ka-thunk” (two syllables) in the F-16 but I definitely heard and felt it in the RF-4C. I remember refueling from the backseat and knowing the boom had contact by the feel and the “ka-thunk” (couldn’t see the director lights while backseat refueling, but could see them if the guy in front was.) And as an aside, I can say with complete confidence that until I had a dozen or 30 AAR hookups in the RF-4, I was always squeezing the shit out of stick grip. Then one day a very cool WSO says, “Hey mind if I try.” WSOs were NOT permitted to refuel. I gave him the jet and he smoothly drove in, took a thousand pounds, disconnected, slid back to pre-contact, said “Thanks, your aerospace training device” all the while humming. And magically it was like ALL the self-inflicted pressure was gone, even with all the trimming while on the boom and often having to put one throttle in minimum burner and the opposite less than Mil to stay on the boom. In comparison refueling the F-16 was an absolute piece of cake.
 Regarding a two syllable sound, I suppose that might have been a result of the boom making contact with the fuselage spine (RF-4) or IFR door (F-16) and then sliding into the receptacle. It was quite common to find rub marks around the IFR door where the boom had made contact. So if the boomer simply stabbed/plugged you cleanly it would just be “thunk.”
Don’t take my word for the feel and sound in the F-16, see what Fox3TwoShip says.