I have not experienced a noise like that, but it sounds like the front pump in the transmission. If you hear it on acceleration (which you don't) then it is bad. Have you checked the transmission oil level? The oil should be red, clear and lightly scented. It should not be a brown colour and smell strong.
Exhaust can make strange noises under deceleration. Have you checked the heat shielding on the exhaust vibrating or the exhaust touching something, possibly due to excess movement of an engine mount. | 
