Engine rattle stopped when clutch failed

Does your car have a manual transmission?
If so it could possibly be your throwout bearing.
That can usually be checked by slightly pressing on the clutch pedal. If the sound goes away then it is definitely your throwout bearing

resonance at rpm when stationary can only come from those parts rotating
That limits the noise source down to the engine and transmission
Starting with the engine , the most obvious noise sources are the idler pulley bearings , water pump bearings , power steering pump, ac compressor pulley bearings (compressor if activated), drive belt-loose or failing and harmonic balancer rubber failing
There may be a noise source from a worn/loose timing chain and that noise will be as a harmonic rattle
moving on to the transmission
If manual , bearing noise can be determined by the application of the clutch
Throw out bearing noise will stop or get worse with the clutch depressed and gear /bearing noise will stop with the clutch depressed
Probably not a lot of help as the description of resonance is wide open to harmonic--rattle , whirring, hum or even loose material in the exhaust system such as failed cat converter moving around

the noise when in neutral is a bearing noise
it can be from a failing throw out bearing that is rattling until you put pressure on it ( will have to be replaced eventually) or an input shaft into the gear box which will have to be replaced sooner rather than later
IF it is hydraulic system with the engine idling see if you can manually pull the release lever away from the throw out bearing ( pushing the clutch slave piston back into the cylinder. If the noise stops it will be a throw out bearing
unfortunately if the slave cylinder is a concentric type (inside the bell housing )and not external you will not be able to test it

Sounds like a release bearing is going bad.

That may be the clutch release bearing-a part of the clutch assembly inside the bell housing. If that is what is humming, it will be okay for awhile-maybe a few months. Eventually the noise will get louder and become a screeching, drive-you-crazy noise ! Then it will be time to replace it-a small donut-looking device with a sealed bearing inside it. Apparently the bearing is starting to fail. Have a mechanic listen to it to verify if it is the release bearing.

I would be checking the throwout bearing. But, if the clutch isn't slipping, maybe leave it for now?

If you put it in 5th gear, let out the clutch with handbrake applied and it stalls--the clutch isn't slipping for sure.

Sounds like the bearing that pushes the clutch arms to release clutch or the clutch itself is B/O

it maybe the throus bearing on its way out

1st to do,check transmission oil level

Check your gearbox oil level.If thats ok.Then you have a problem with the thrust bearing either worn or dry.The only solution is to fit a new clutch kit.Hope that was helpfull