First I'll assume you checked everything visible to mack sure it was tight.
So that leaves the "rattling" noise to be internal.
That could be three things that I can think of quickly off the top of my head.
- Bearings
- Piston/Ring(s)
- Piston Rod [connecting rod]
Two sets of bearings would be behind the black end caps of the motor. Another set would be directly under the orange/red cylinder head down in-line with the center of the motor.
The piston & piston ring(s) would be under the orange/red cylinder head.
The connecting rod will be below that.
With that said, try to identify if the rattle is coming from any of those areas. If so, then the cost and availability of parts becomes an issue. You might find it feasible to purchase another compressor and mount it to your tank.
Or, maybe there is an oil fill hole and it needs some?
Good luck.