First post, had a few questions

Just bought a 01 325xi a month ago, guy had a lot of work done on the car, receipts for everything had new valve cover axles pretty much the works. Came with enkei rims new Yokohama weather tech mats and new spare parts 02 sensors filters bulbs ect. Paid 2k for the car. After a few weeks noticed when I switch to shift mode sometimes I lose power random in 1st and or 2nd but after that it's fine and it's always on take off, wonder if u guys know what it could be. Also while I'm driving I hear almost like the exhaust is loose or a plate is vibrating any ideas on that would be helpful thanks

Change the trans fluid & filter-No such thing as "lifetime fluid". Not the easiest access but it should solve issues.
As far as the rattles look for lose heat shields and worn hangers.