On a recent trip,(about 2000 miles), my 2003 6.oL powerstroke started an intermittent noise while pulling my 7x24 ft cargo trailer. Was very heavily loaded for half the trip and pretty much empty on the second half. Noise seems to come from left side of engine and sounds like a locamotive trying to get up to speed but has more of a growl to it, but the steady wha wha wha. Sometimes no change in engine behavior, other times with small rpm drop and surge with pufffs of black smoke out the pipes corosponding to the surges. Sometimes changing rpm will help or worsen the problem. I have talked to a Ford dealer and a heavy truck mechanic with no clues from either. This action seems to start after about 300 miles of running. It may quit and run fine and usually does after it sits for awhile(overnight). It doesn't seem to do it if I am not towing, so far, or it is so minor it is hard to detect. Truck has 38,000 miles. 37K when this started.
Please offer any advice as to where to start troubleshooting.