engine with bad valve rattle

I need some help. I have a well maintained 150 2011 with 39000 miles. It has in the last 5000 mile developed a mean valve rattle. The dealership has taken the "we have not been able to reproduce the problem" approach. It is diffently a lifter or valve rattle that normally occurs in 5th or 6th gear when RPM's are at 900-1200 with speeds around 40-50 MPH. I can make it worse by feathering the gas pedal. The dealership had it the truck a month ago and at the time i had not yet figured out how to reproduce the problem. I actually stopped in one afternoon and the damn thing would not do it but like I said i had not figured out how to make the prolem sound off.
I think I can make the noise happen with my second visit to the dealership but wanted to see if any one else has experienced a similar problem.
Thanks in advance