I had this issue after I changed my Diff fluid and switched to Riddler covers. I was following directions on how to change diff fluid (directions from "another" forum) and it said use 2 quarts. The driections were for the rear diff but then went on to say, repeat above steps for the front.
Stupid me...I way overfilled the front and did not know. Started getting that rancid smell and thought it was bad exhaust or something. Was very noticeable at a stop and with my vents blowing.
Took it to the dealer and within 2 minutes they started laughing and asked if I changed my own diff fluid recently. It was pouring out of the breather tubes in the front!
Easy enough to fix but now I am very careful about how much I put in.
For the record, it appears that with my covers I can put in about 2 1/4 quarts for the rear and 1 1/4 to 1 1/3 in the front.
Hopefully your issue is just that and not something bigger.
2010 Black Unlimited Rubicon w/NAV
Superior 5.13 gears
2.5" Teraflex coil lift
Bilstein 5100 shocks
JKS front and rear adjustable track bars
LOD Signature series rock sliders
35" Maxxis Bighorn Tires on 16" blackrocks
Terafkex C Gussets and inner sleeves
Rugged Ridge XHD Bumper
Smittybilt XRC10 Comp winch
Rock Hard Evap Skid
Smittybilt seat covers (sitting in garage)
Mastercraft Grab Handles