System ( +25 yrs ) is short cycling. Sound at cold air returns closest to furnace sounds like someone sucking for air ( my Dad had COPD and it sounds like him.)

Contractor observed:
Plenum super hot on the blower side; plenum much cooler on side nearer to panel.
Blower motor dirty.
Inadequate cold air return -- suggested adding a return by cutting into sheet metal on blower side of furnace.
Heat registers on the INSIDE walls; returns on the outside walls --- house has been this 'ducted' this way from construction 50 years ago.
With the panel door off by the blower furnace runs longer; with panel back on running 2 minutes.

All was well with this furnace the past 3 years.
? Short cycle, why--- more ideas ?
Why would the plenum be SUPER hot on side ( contractor says the limit switch is turning off furnace as designed )

Any suggestions, comments...

Contractor nailed it. Lack of airflow. Either plugged filter, undersized return, plugged evap coil(if central ac), undersized supply(or supply register are covered/blocked) weak blower moter, extremely dirty blower wheel, loose belt(if belt drive blower, and on a twenty five year old furnace, it may be). Or possibly on a furnace that old, it may be a bad fan limit control, allowing furnace to get to hot before control kicks on the blower. Lots of options here.

Now. With the sucking sound you are hearing from return grill, it sounds to me like lack of return, or maybe a return somewhere that has a couch or bed or clothes or something blocking other return. Make sure all returns are clear and all supplys are open and clear. Let me know what you find.