Repair Question - In need of some real help


I have a 2001 325I with 98,000 miles and i was told by BMW that i need to replace my torque converter [:0]

Im not so sure if they are correct so im here to get the advice from the people that drive and love these cars... cause if they are right then im going to just sell it and since im so close to finally paying it off that is something i really dont want to do

Anyway ... The sound it makes is a low bassy sound that only comes when driving at a stead 30-40 miles a hour and holding it there for a few then it almost sounds like im driving over those bumpy road strips or like a low bass sound coming from the back end of the car it will last for about 10 seconds and come back again in about 2 mins
Also when im on the highway and hit 60mph the tramission seems to slip or buck for that second when it is getting over 60mph then it drives fine.

the 1st BMW guy said it was the driveshaft and then they changed there mind and now say it is the torque converter.. Im confused and im not sure who is right ... they want $2,600 to repair it and i cant really offord it :-(
the check engine light is NOT on or anything so can they be wrong

please help .. what does it sound like is wrong to you ...??