Here's an odd one.
I had the crankshaft damper pulley fail on my 2003 ZT diesel last year, accompanied by the usual rattling / clacking noises. No problem, replaced it with an aftermarket from Euro Car Parts. At the same time, I replaced the air con & ancillary drive belts with a couple of the Continental made ones.
The pulley rattle disappeared, but now had a metallic rumble / clack at around 1200rpm, give or take 200rpm. Not as heavy as a crank pulley failing, or starting to, but a very similar noise & heavy enough to turn heads at a zebra crossing.
Although they felt OK, I ended up replacing the idler & tensioner pulleys, as what they feel like when turned by hand is vastly different from what they do under load at 1000rpm or more. No better. Notice a slight water pump leak when doing the pulleys, so replaced this also. Still no better. The noise did seem to be coming from the front lower area of the engine, but very difficult to tell. Replaced the power steering pump, but no joy. I then thought I'd been sold a sub OEM quality or faulty crankshaft pulley, so bought another new one. Still the noise persisted.
I was now beginning to wonder if the original crank pulley failure had caused damage to the 1st main crankshaft bearing, because of out of balance spinning of the original pulley when it failed. Very unlikely, but I was now in the realms of straw clutching. Bought a stethoscope & had a good listen around the engine. Very loud around the alternator(ish) area with the naked ear, but not much through the scope, although scope was loudest on the alternator, this noise didn't seem to be from a fault there.
With all this messing around, the ancillary belt had started to chirp, so in an effort to try & quieten it, dusted it with talcum powder. Chirp disappeared & so did the rumble!
On looking at the belt, it 'bounced' up & down with a total deviation of around 15mm or so at about 1200rpm, running fairly true above & below this speed between the crank pulley & alternator pulley.
By now, these belts were getting on for 15k miles old & a lot of man handling & greasy hands on them through the fitting of the new parts at various times, so replaced them.
This time I bought a QH ancillary belt QBR52025, which is 5mm shorter & slightly thicker than the Continental.
The problem disappeared.
I was a mechanic for many years & although not my job for a good few years now, I still keep my hand in, but this is the 1st time I've experienced or heard of this. I would never have thought a dodgy belt could have caused such a heavy metallic noise. The best way I can describe the noise would be like stretching a bungey cord tightly over a 5 gallon metal drum & plucking it rapidly. The only answer I have come up with, is that the 'twanging' of the drive belt was transferred up through the alternator & the alternator casing acted as a resonator box, behaving a bit like guitar.
The whole experience cost me Â£325 in new parts & tools sturdy enough to do the job, so the morale of this story is, if you are sure its not your crankshaft pulley & you can see nothing else obviously wrong, try the belts 1st, its the cheapest (& easiest) option.