I know you've asked this question several times and gotten conflicting answers. As you know, spring reverb units have a couple of springs mounted side-by-side in a long rectangular metal box, usually mounted on the bottom inside your amp cabinet or head. When the amp is shaken, those springs will flop around, and sometimes hit each other or the housing. If the amp is turned on when this happens, you get a crash through the speaker. When the amp is off, the same action can produce a sound that might be described as a rattle.
Some reverb units are more prone to this than others...it depends on the size of the housing, the clearance and tension of the springs. This is probably why you're getting different answers here.
I've played guitar for over 46 years and have played through many amps with spring reverb units. Some of them are very quiet...you have to drop them off stage to make them rattle. I have a Peavey Session 400 that is so sensitive that I can't put it near the drummer on most stages. I can hear the springs rattling in that one when I carry it around. You could test this on yours by turning the amp on and setting the volume real low. Reproduce the sound by moving the head and see if the rattle corresponds with reverb crash through the speakers.
There really isn't any other part in an amp that would sound like a spring rattling, even if it was loose. If you're really concerned, you should bring it back to the seller and let them check it out. Perhaps they have another Marshall head and can demonstrate that it makes the same sound...or not. Ultimately, I think you won't be satisfied until you've done this.