[beenthere]

If the air filter is being drawn in towrad teh blower.
Its a sign that your return system is too small.

Your HVAC buddy can show how to get to the coil to clean it.

 

[beenthere]

Look for screws on the side.

 

[beenthere]

The plug is "plugging" the secondary drain opening.

You'll need to remove the flue pipe to get to the coil. To remove the plate on the coil yet.

 

[beenthere]

When you remove the plate that you currently see.
You'll see the a coil. But only the discharge side. Not the bottom side where the dirt will be.

There is another plate on the coil, that need to be removed to get to the inlet of the coil.

The cloth duct tape is good for everything BUT duct work.

You should be able t get mastis at Lowes or Home Depot.