Hey guys,

I have a 2015 R RWD, and under hard acceleration, there's a big smack either between the seats or right behind me, especially in 1st gear. Doesn't sound mechanical or related to the diff (fluid recently changed) and definitely doesn't sound engine related, but it kind of sounds like a speaker loose or something back there. Any ideas? Also, is there an easy way to access the area behind the seats without a bunch of tools?

Thanks!

-C

 

Have you checked that both batteries are tightly secured? (The 2015 car has two batteries, principal battery to the left and stop/start battery to the right).

 

I have a 2015 R RWD as well and mine developed a single hard knock when changing from reverse to forward movement. It was caused by excessive play in the splines where the drive shaft input enters the rear diff. It was fixed (via TSB) by adding extra Loctite to the splines. It was most obvious and 100% repeatable when changing from reversing to pulling away in drive quickly (as in the change was quick, not the driving speed).

I mention this because to me it initially sounded like a rubber mallet hitting the front wall of the trunk space just behind the seats at around bum height. Imagine a bowling bowl loose in the trunk that rolled forward and hit the wall. With the trunk floor carpet and parcel shelf fully removed from the car it was a bit more obvious the sound was coming from the diff area. I asked the dealer to check for a loose diff mounting because that is exactly what it sounded like with the carpets removed, and then they found the TSB about adding Loctite to the splines.

It was 100% reproducible for me so if you do the steps above and it doesn't happen every time it probably isn't the same cause, but I wanted to mention it just in case. Likewise if it happens without changing direction.

 

Never heard of Loctite being used on splines. There was a TSB on installation of isolator sleeves to be installed on rear springs of 2014MY for same diagnosis. Might have included early 2015 models also.

 

JTB00402V3 is the one that was applied to mine.
There doesn't appear to be a VIN cut off so it is likely a change that was adopted into the build process at some point.

 

I checked this morning, and they both appeared to be secure!

 

This is super, super helpful. Pretty sure that's EXACTLY what's happening with mine, and it's 100% repeatable. It feels like a mallet slapping me in the back with a slight bump in the rear as if I ran something over when you really step on it in first gear.

Woot! Glad to know it's not something too crazy and will attempt to get this addressed during the week.