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Why would that surprise you? When the truck is running, I'm sure the truck's alternator, ignition coils (if it's gas) and other moving parts are putting out a pretty decent electro-magnetic signal. Even when it's not running, that much metal and wiring is sure to absorb, reflect, or otherwise screw up a signal.
(I was a Field Radio Operator in the USMC - Mr. Murphy is our constant companion.)
Barrister - I believe the answer you're looking for is here.
One size fits all. :)
We presume this is with the truck turned off, right? If it's happening when it's running, replace the high-tension ignition wires.
Otherwise, plan on moving something. Is the wireless a card or a remote antenna (like my Verizon Wireless) that's hooked to the 'puter with a USB cable? If the latter, you can buy a USB 'extension cord' that uses the 2v from the USB jack to power the signal so you don't lose anything.
If it's a card, and presuming you don't want to move the whole computer, then you've got a prob. Have you tried backing the truck in?
If the problem persists and there's no other way out, your tin-foil hat suggestion (aka a 'Faraday cage') might just be the ticket. Line the wall between the computer and the truck with sheets of sheet metal and ground them to the nearest water pipe. Tell everybody it's a "radiant energy collector", just like a solar collector except it scoops invisible radiant energy out of the air to power your computer. Who's to know?
Probably not emitted noise from the ignition, alternator, etc on the truck - the frequencies that the wireless uses are way above where these emit.
Could be a computer module on the truck - lots of controllers these days for ignition, brakes, etc.
Most likely, however, is reflections. Multipath distortion is a big problem in wireless - that's where you receive both the direct signal from the transmitting antenna and a reflected signal that's also pretty strong. The reflected signal has a longer path to travel, and so arrives slightly later (about 1 nanosecond per foot traveled). This can cause major inteference.
So my guess would be that there's enough metal there to cause some reflections.
Note that the Faraday cage idea might work, or might make it even worse because of the same factors.
I'm not sure if this will shed any light on you're question, but sometimes when I'm driving in my old pickup listening to am radio and a truck passes by, their CB conversations come over the radio but sound like an upper harmonic of the original sound.