Yes......the cooling fan is a real conversation stopper when it comes on above 3000rpm's. I have improved on that and it is a boatload of work, here is what I have done.
So I have completed my cab noise control project for now with truly pleasant results.
First I ordered some 1/2" 4lb noise attenuating foam from the thefoamfactory.com.
It comes about 70" wide and is sold by the foot. I cut a piece to fit in the cab to house bellows area cut as wide as I could and still snap down the cover. This made a difference in the noise coming from that area, good start. Next I picked up some 3M Professional Rubberized undercoating (6 cans) and coated the fender wells and under the cab with the exception of the engine area behind the radiator, not that that area did not need attention but I was unwilling to coat the area around turbo with a substance that is possibly flammable. The next step was a boat load of work. Thanks to Steven Pon I took apart the base of the cab (with a full days help from Steven) , seats out, console out and all floor material out. The 1/2" foam was placed under the whole seat area from the top of where the vinyl floor material starts at the back of the cab to the edge of the floor below the seats. We used a heat gun to warm and soften the foam sheet and pressed it into the contours of the cab bottom (heat the foam to soften press into the contours with wooden tools made for the job and hold until cooled[pains taking to say the least}). When the whole bottom was covered we used a 7/16" punch to make the holes for all the attachment points for the console, seats and seat belts etc.. The vinyl and seats and console were all replaces (much easier said than done, needed to get longer bolts in many cases). That leaves the floor area. For the floor area I ordered DynaPad (made by Dynamat) from Amazon. This is a multilayer noise control foam that is 1lb/sq.ft., heavy stuff. This material was placed on the floor starting at the top of the sloped area in front of the base of the seats and extended across the floor and up to the "firewall". This material comes 54"x32"X0.425" and was used as a single piece centered in the cab and cut around all the bits in the way under the dash. This left small areas on either side uncovered, I cut leftover pieces of the 1/2" foam to fill in this area and attached it to the DynaPad with Gorilla duct tape. All done, I replaced the floor vinyl, the plastic sill pieces and the floor matts. Time for a test ride. Jeanne, Mustard and I jumped in and headed to the highway for a test drive. At 60mph with the cooling fan on turning 3500rpm's (dropped into 5th when the fan came on) conversation was possible! The general noise level in the cab was noticeably lower. A success!
What would I do different? I would use all DynaPad, the material is easier to work with and is designed to control the low frequency sounds that are generated by the cooling fan (and other engine and road noises). This was a total of 3 days, 8 hours/day of work, 24 manhours (Steven for one day) and for me totally worth it!
The other opportunity that I see for noise control is the front wall of the house, the rear of the engine bay is wide open and focused at that flat surface, reflecting the noise to the cab. This would require a waterproof noise attenuating material to cover the area, and because of the proximity of the turbo exit pipe the material would need to be fire rated. I am looking into a material that will meet that spec. I will report what I find.
Any questions feel free to contact me 508-820-6430
Happy Trails,
Glenn C. EC34 Wolnosc