So I am a recent second owner of a 2012 BMW 128i. I am not new to BMW and consider myself a fairly good hack mechanic - lots of experience maintaining an e91 wagon prone to numerous failures and need for maintenance. So enough of that. I noticed this clicking sound too. My 1er is a manual and I noticed the nose in 2nd gear to 6. When I lift of the accelerator at low RPMS (approximately 3k), I can appreciate a very distinct click - as if something is engaging or even disengaging. It even happens when I feather the pedal under little to no input - this causes multiple clicks. Since my car remains under warranty, I took it to the dealer. The service tech took me out and was completely unenthused and attempted to write it off as possible plastic rattling. I am no dummy, it was not plastic. So they took the car in to the shop, and after further examination decided that the noise was a defect in the exhaust. So, they replaced the exhaust. I picked up the car and quickly discovered that the exhaust replacement did not solve the issue. It was now very logical and likely that the noise was not the exhaust, but something else. My ears lead me to believe the noise was possibly under the rear seats or somewhere in the trunk. So I decided to remove the trunk board and put the rear seats down. After doing this, the noise was much more amplified and audible near the battery.
I recently came across
http://www.1addicts.com/goodiesforyo...le_Manual1.PDF. On page 27 of the document, it references, "Intelligent Alternator Control (IGR) is also referred to as Brake Energy Regeneration (BER)."
I quote, "When the driver lifts off the accelerator pedal and steps on the brake the system uses that otherwise wasted kinetic energy generated as the vehicle is stopped to replenish the battery state of charge. Ultimately as the driver applies the gas pedal the charging system is disabled to render the engines maximum power."
I believe this is the answer to our enigma. I am not an engineer, albeit per the above reference, it sounds beyond a reasonable doubt, as if the clicking is totally normal with the BER system. I hope this helps.