1996 Civic DX Coupe 77K

I have noticed a knocking noise from the engine at 35-40 mph.

No loss of power or gas mileage, just a strange
"chugga-chugga-nok-nok-nok" at 35-40 mph. Its not very loud, but
notciable.

Any guesses? Clogged injectors?

Last major service was at 60K (rotor/distcap/plugs/wires..etc)

Any help appreciated.

Thank you all.

-Slim