take a block of wood and place on a jack - place the jack under the engine and very slightly raise it - if the noise goes away then check the engine mounts.
I have repaired such a noise problem in a margalla, but it was not related to any fuel, I removed the exhaust from the engine and the middle of the car (original flange bolt system) - I put the exhaust back so it was not under any tension and aligned the entire exhaust system so it sat "neutral" - the noise disappeared. That noise was a booming hum that raised with engine speed.
Do note that all A/C compressors make a very slight noise when working - it is loudest when it switches on.