There are so many possibilities!
loose exhaust
Clutch throw out bearing bad.
Serious engine knock or valve train noise. Have this looked into by a pro.
Most probably the noise is piston slap, this noise is heard when the engine is cold and disappears when the engine is hot. if the slap is very strong when the engine is cold and the noise becomes less when hot but still the noise can be heard.
In most cases, you can tell that your lifters are bad, in your car engine, by the ticking noise. A bad lifter will make a tapping sound.
it could be your driveshaft. I'd get it looked at if i vvere you.
When you can not figure out why your car makes a strange noise, you take it to a mechanic.
over 55 mph
If it was overheating at the time, you cracked the block.
Oh bummer. I had one and it was the rod bearings. You have to have the engine rebuilt and I didn't do it because it was going to cost more than the car was worth. Maybe your noise isn't so serious, but it will cost you some money to get it fixed.
if you turn the key and nothing is making noise check the battery if you are trying to start it and it is making noise press the gas a little bit so that the car can pump gas into the engine
exhaust sytem making contact with underside or engine mount damaged/worn, the sound will resonate inside the car but not outside.