All carb models are mechanical shift only and that includes Volvo Penta as well. There is no alarm for shifting in gear or out of gear on any carb models.

Are you sure that its an engine alarm ?

you are going to have 4 warning horns period with Stock Merc carb app here.

1 is key on only / and or starting engine - it goes off when when oil pressure switch sees 6 psi upon start up

here is the other 3 warning horns ONLY for carb --- and Carb TKS sterndrive models

2, Oil pressure

3. Engine temp

4. drive lube bottle

Without seeing the app my guess is you have a bad oil pressure switch and or related in the mix possible too low rpms when shifted in gear inwhich decreases rpms and then causes lower oil pressure. In N gear idle rpms are higher than in gear inwhich oil pressure is raised.

Also with too low rpms comes decreased water flow so with that said what temps are you running ?