The van runs great. AC works great. All fluids have been changed, always maintained by dealership. The train horn sound comes from the air filter area , or so it seems, and can be very loud outside the vehicle. Only once the vehicle has been driven for a while. No rough idling, no rough running, just an annoying train like sound. Stops as soon as the accelerator is applied, comes back as soon as idle is reached.

I was told by a so called mechanic, it was the mass air flow sensor. I changed the air sensor, plus a new air filter. I just recently purchased the van from the original owner. It was a garage queen, maintained by Ford dealer. I'm just looking for advice from someone who may have experienced this noise, and what the fix was/is, thanks.