NOTE... this sound is most likely NOT coming from your heater box or glove compartment.

Listen closely and make sure it's NOT your stereo head unit. Mine does this from time to time as it tries to read the disc in the unit. I have the head unit with NAV.

It sounds like a "TAK-TAK-TAK-TAK-TAK-TAK-TAK" sound (if that describes it good enough).

Sometimes long... sometimes for only a half second or so.

I just let my system ride through it. Only once have I gotten a "Disc cannot be read" error on the head unit. I simply ejected the disc, removed it and then re-inserted it and let the unit take it. Seated and spun up right away.

Hope that helps.