New 530i owner - Vibration??

[Stan]

I recently purchased a 95' 530i with an automatic transmission and about 72,000 miles. I'm new to BMW and haven't had a rear wheel drive car in almost 20 yrs. The car seems to have been well maintained and drives well, but I have noticed a slight vibration that seems to be coming from the drivetrain as the car shifts from 4th to 5th gear. It lasts only a second and sometimes isn't even noticable. Is this something that needs attention like a drive shaft bearing or is it "nature of the beast"??? Could it be a transmission issue??? I don't have maintenance records so I don't know if the trans was serviced. Any help would be greatly appreciated.