Oh, hitting, not plucking, would make the sound. The timbrel appears to be a tambourine without the drum head from most of the photographs I checked out--or perhaps simply a very small tambourine. Since it's called the timbrel and lacks the 'tambour' or drum component, I would suspect that timbrels were simply circles of small metal pieces hitting against each other and that the tambourine was distinguished from the timbrel because of the drum head addition.

It would be interesting to know the history of the two instruments. I'd guess that the timbrel preceded the tambourine. The timbrel is mentioned in biblical text.