Slight grind in tow/haul 3rd to 2nd shift.

I noticed last night that if I used the tow haul setting under normal easy driving conditions I heard a very faint low level grinding or growling sound when the trans would shift from 3rd to 2nd under easy normal braking. The radio has to be off and you have to listen for it but it is there. It does not do it in regular mode or od off just tow haul. I thought it may be the higher line pressure forcing a quicker down shift or somethng to do with the more aggresive shifting of the tow haul mode. Has anyone else noticed this or is there anyone with some info on what it could be.
Thanks Marc