03, f350srw, 6.0 noise while pulling

I have an 03 F350 srw, 4x4, auto, 108500 miles, which has started to make a noise while towing up a hill. My 5th wheel trailer weighs 8500 lbs. The noise isn't constant. It will make the noise while under strain, pulling up hills, but not all the time. More of a wooshing noise as though something is blocking the airflow, releasing and blocking again. I have noticed a slight lose of power when this happens. New air cleaner made no difference. Any idea where to start to correct the problem?:runaround:

got this from http://www.internationalpowerstroke.com/6.0_problems.html
The EGR might be something to check.
The EGR coolers on the 6.0 liter Power Stroke are prone to clogging due to oil & soot build up, which can cause them fail. A failed EGR cooler may contribute to high coolant temperatures of coolant loss. EGR valves are also notorious for failing due to soot build up, which can lead to over-boosting of the turbocharger. To prevent this, the EGR valve should be cleaned routinely. Excessive idling and poor fuel quality can clog the EGR valve rapidly. The efficiency of a diesel engine is reduced at idle due to low combustion temperatures, resulting in increased particulate production.

I disconnected the EGR Valve and it ran great. I cleaned the EGR yesterday morning, it ran great for a while. Then it started the same thing as I was going down the interstate. I pulled over, disconnected the EGR and it ran great again. This happened while NOT pulling the trailer. Any suggestions?:runaround: