2002 Tacoma Transmission issue ?

[rbarr110]

I have an 02 Tacoma that I bout new in 2002. I recently have had what I believe is a transmission issue. ( I am not very good with engines but am hoping to get ideas on what the problem is before taking it in for service work).

The tacoma is an automatic and is not 4WD (live in the city, wanted a truck for light duty household hauling). When I drive it and get to about 25-30 MPH and idle around that speed it sounds as if it isnt switching gears or is stuck between gears. It makes a sound similiar to when you have a hole in the muffler and rattles somewhat. If I push the overdrive button, I never have the issue.

Any ideas on what is going on?