Hi, after I start my 2009 XF SC I get about 30 seconds of gentle knocking from the transmission after it is selected. It sounds like piston slap, but interestingly only in drive... it goes away in neutral, or after about 30 seconds to a minute. You can't hear it outside the car, just inside with the windows up.. am I brewing up a problem, or is it something to forget about??.... It has done 60,000 miles, and is problem free... any thoughts?