I have a 1993 Nissan Sentra its a manual and has about 190K miles and every now and then its makes a really low almost motorctcle like noise, its has done it when the gas is low, but it doesnt have to be. Its not a conitnuous noise it can go away after you drive a little while and it can also just start making the noise while driving. When it is making this noise it uses A LOT of gas. Any Ideas on what the problem is and how to fix it?
Seems like a great amount of unburned gas is getting into the exhaust manifold or even farther down the exhaust, and burning there, producing a loud sound. Probably some does not burn, and ends up in the exhaust gasses and maybe in your oil. Check the oil for gassy smell and/or too-high level. Excess HC is easily measured at the tailpipe with the right instrument.
Maybe the fuel pressure regulator is not working. Fuel at a very high pressure (therefore volume) gets past the injectors. Having the fuel pressure measured downstream from the regulator would be an early step in diagnosing this problem.
The catalytic converter can be ruined by excess unburned fuel.