Q: BMW 330d Rattling-Hissing Sound from exhaust side

I have a 2004 bmw 330d automatic (steptronic) with an M57N (M57TU30D) engine and is making a rattling, squeaky-hissing sound from the exhaust side of the engine on idle. Started off a a minor faint sound but is now louder and sounds like a worn out clutch/ DMF. Sound is not coming out of the engine itself but outside towards the tranny. When the problem was getting worse, I was getting a yellow oil light after switching off engine and responded by topping up with a pint of oil but the light still comes on. Checking the dipstick would give me a normal level so I am no longer topping oil with the fear of adding too much oil. I also have acceleration and power issues when going uphill and have to resort to changing to manual shifting so that I can get enough power. I am inclined to think it could be vacuum-related but want second opinion and confirmation. Thanks in advance
My car has an automatic transmission.