This sound clip is a Yanmar Marine 1-cylinder 4-stroke reciprocating diesel engine at idle. The signal was recorded with a GSM cell phone through the Audioblogger system.
The baudline FFT analyzer created the time-frequency spectrogram image below:
Baudline's periodicity bars were used to measure an accurate 0.121 second delta between pulses. This works out to 8.26 pulses/second or 496 PPM (Pulses Per Minute). Since the Yanmar is a 4-stroke reciprocating engine, multiplication by 2 will calculate RPM. So: 496 * 2 = 992 RPM which is close to the engine's idle warm-up speed.
A section of spectrogram data was copy-n-pasted into the average spectrum window shown below:
There are two strong peaks of unknown source at 230 Hz and 300 Hz. The exhaust note is an unlikely candidate since it is piped outside the cabin. The air intake and/or the hull resonance are suspected. The recording does not convey the loud pounding sound of the 1-cylinder engine which could be mechanical in nature. The two strong peaks are likely a combination of a couple of the previously mentioned signal sources.