I have an air return in my ceiling 20 X 25 which I recently replaced the filter in. After about a week the new filter started a loud vibration. I figured it was not the exact size so picked up another one and replaced it and the vibration started again. I have narrowed it down, to it only happening when a filter is placed in the hole. If there is no filter than there is no vibration. Which areas, should I investigate to see where the vibration is coming from. Thanks so much for the help.

In the return grill the filter does not or should not fit up against the "hole" or top of return air can, there is a lip at the bottom near the grill that keeps the filter from going up to the top. If the filter is at the top it actually can restrict the air flow because the air can only pass through the filter where the "hole" is. Hope this helps, Mike