No change in whistle with different fan speeds or HVAC control to "off". The viscous damped clutch for the cooling fan was replaced.( had a "roaring" noise ). Last week I found a keg out of a 5.9 and did the "mod" as well as fabricating a new belly pan from 1/4" aluminum plate. Installed it on Friday (removed both front seats to work on it- what a pig of a job). The van seems to run smoother and has only whistled faintly a couple of times since. First chance I get , I'll pull the dog-house and double check the torque on the manifold bolts. Vacuum seems to be about the same, too soon to tell if it's helped my mileage. I've got my fingers crossed.