Noticing a burb, burb burb sound in the left side towards the rear of my '94 E320 Cabriolet. It only occurs when the car is hot, after a distance of highway driving, upon re-accelerating (like on off-ramps). Seems that the noise is more audible when re-accelerating at "gentle" speeds; it's not particularly obvious when re-accelerating at higher speeds.

Have eyeballed the area around the tire & wheel & can't see anything that looks worn or loose. Any ideas on what this might be?