one of my 496 h.o s keeps beeping twice a minute at idle, with the engine speed jumping about 300-400 each time. Does this about twice a minute under 1200 rpm, but runs fine over 1200 rpm. Merc scanner shows no codes, and everything normal. Does not go into guardian mode either. Tried a new iac, new batteries, and no luck. A little stumped here and looking for help . Thanks guys.

Check owners manual for a list of reasons for that type beep. Could be drive fluid level.

+1. Drive lube monitor. Low drive lube level.

drive lube warning won't cause a 300-400 rpm jump. Low drive lube would also show up as error on scanner.

It would for me, but then again I'm usually in the process of breaking a drive.

My guess as well....

Why would low drive fuild cause the rpm's to jump? I don't see the correlation there.

check your impeller in your water pump,,, there is a water press sensor in the pump,,,, try just unplugging it first with the engine off and if the beeping stops,,,, then thats probably it,,,the rpm could be a piece of the impeller binding in the pump,,,, if its an older 496 HO it could be the sensors in the Exhaust manifolds,,, again unplug and restart the engine

Is the alternator charging good under 1200 rpms?

My educated guess here is that you may have either some loose or slightly (corroded) bad ground wire connections or a bad or loose sensor connection on the harness at the sensors that is possibly intermittent. Try unplugging all sensors one by one and spraying some contact cleaner in each connection and clean and tighten all ground connections on engine.
Also since the idle is jumping up is this actually happening on the engine or are you just reading the jump up at your tach? This jump in rpm if at the tach only suggests the bad ground connection.
You can also consult the Merc diagnostic manual for the PCM555 and see which faults show up as that beep period and sequence.
Don't worry as you will find the culprit.

Best Regards,
Ray @ Raylar