Astro Mini van! HELPP!

I have an 1993 astro mini van which has 186,000 miles on it. Its an automatic transmission and 4 wheel drive. There is one strange sound that nobody knows what it is. It sounds like the wheel bearing but I had them all changed & it only does it when your going past 35 miles per hour & rarely will do it in overdrive. It sounds like a humming noise sounds like its coming from the passenger side dashboard. There is no fluid leaks, which I just replaced the water pump last night. But the problem is the air filter has not been changed for a very long time. My boyfriend & Dad looked at it & realized you have to take the dashboard off to get to it. The guys at monro & wal*mart said the same thing. When we took the dashboard off there was no sign of an air filter. I need to change it bad but do not know how. Can you please help? :(