The ejector pump recently started cycling every 20 minutes, and iS louder than ever before.
I replaced check valve, but still same problem. There is no bathroom in basement.
Discharge pipe is 1-1/2" and runs to larger PVC that flows into septic tank.
Waiting for some experienced views before removing sealed cover for visual.
Thanks, Dave

What drains to the pit if you have no bathroom down at that level

I believe drainage from wash machine, utilty tub (which is rarely used), and possibly kitchen and /or bathroom sinks.

Open the cover and observe how high the water gets before the pump turns on. The float should rise several inches before iot trips the pump. This could be a float malfunction or just an adjustment

Or it could be a blocked discharge pipe with the pump running until it over heats. Then coming on again when it has cooled. If so, you will not see any difference in the water level as the pump runs.

What can I expect when removing cover in terms of smell.
Am I releasing amount of methane?

If it collects what you said there will be a sewer gas smell but if the pit is vented correctly to the roof it will not be so bad. Yes some methane along with about 7 other gasses but short term exposure at this level will not harm you.

What can I expect when removing cover in terms of smell..

Stink.

No, matter has not been there long enough to begin to decay (I hope).

Hi Dave:

Any fixtures backing up... not draining? This would be a clear indicator that the pump discharge line was clogged like Harold suggested. If unsure, run some water and bleach down the utility sink and see if water is pumped out and the sink drains properly. If it does then you have cleared the ejector pit a bit and the odors should be reduced by the bleach you put in the tank and that should help when you open the lid and take a peek inside. May be the float is hung up like Ballenger said... maybe?

Keep us posted...

Mark