Hi

I have a 87' 1 tonne 350 and am undertaking a US roadtrip. We've been on the road for 3 weeks and today it started making a high pitched whining sound like a baloon being let out this alternates with a low hiss like air escaping. The noise only happens when my foot is off the gas (when i'm accelerating the noise stops) I've checked the serpentine belt and it's in good nick. It doesn't actually sound like the noise is coming from the engine at all it sounds more like it's coming from the dash. We've just arrived in Rapid City and it's a long weekend so no one can look at it until tuesday. If anyone could shed some light as to what it may be it would be helpful. We wan't to be able to drive to mt rushmore etc and want to know if we can do it safely. I should mention that the van runs on propane. Thanks in advance