Noises like this are mostly temporary and are caused by a small rock or some other foreign object becoming jammed between the brake disc and the brake pad. Often, a sharp jab of the brake pedal (when it’s safe to do so) will dislodge the object and peace will be restored.

But if you’ve replaced the wheel bearings, then you’ve already had to remove the brakes. So unless the stone or whatever is lodged deep in the brake pad material, that’s probably not the cause here. Which means you need to look more closely.

The ABS sensor is an interesting theory, but I’d imagine if the clip had broken and dived into the front end anywhere, you’d have an ABS warning light on the dashboard by now. Perhaps it’s something to do with the front hub or the front driveshaft assembly. Perhaps a CV joint? Check for damage to the CV joints’ rubber boots. Don’t rule out a pothole having wrecked your wheel alignment, at which point what you’re hearing is a front tyre being torn to shred.

And what about the power steering system. These can squeal when they’re low on fluid, have air in the system, the belt is slipping or the pump itself is worn out.

The trick to a diagnosis here might be to find out what rotational bits and pieces are involved. To do that, you need to work out whether the noise is wheel-speed related, driveshaft-related or engine and transmission-related. The driveshaft spins fastest, followed by the driveshafts and finally the wheels, axles and brakes. So that’s probably the key to it.

Even something as simple as a bent or loose brake shield can cause a rattle, squeak or grinding noise, and some owners have gone so far as to strap GoPro cameras under the car and take it for a drive to see what’s going on. A mechanical workshop will probably be a bit more scientific, but the basics of troubleshooting remain the same.

As far as the driveline locking for a moment, you could be looking at a related problem or a completely separate issue. It’s probably wise to make sure the transfer case isn’t suddenly shifting into neutral or low-range on its own for that split-second, as that could cause driveline chaos. But a sticking brake caliper or faulty ABS module could potentially also cause the same sensation.

Some cars cycle their ABS systems when you first start up and take off, and this can lead to a metallic noise for a second or two and a slight buzz through the car. It can be intermittent, too. Are you sure that’s not what you’re hearing?

Manual gearboxes generally work better/more smoothly when they have some heat in them and the oil gets a little thinner and better able to lubricate. So you’re right on that account. If the gearbox was crunching on every gearchange, you’d also probably suspect the clutch might be worn, too. But since it’s only the one gearchange that’s making the noise, then your theory that the synchromesh rings are worn holds water.

But before you do anything drastic, make absolutely sure the clutch is disengaging fully and that all the clutch lines and pedal action are spot on. A dodgy clutch can also produce this sort of crunching noise. And, yes, it can be worse when the vehicle is cold.

Meanwhile, for some reason, the synchromesh on second gear is the one most likely to fail on the majority of cars (probably because second gear cops and awful hiding and copes with very high loads). So, in that sense, yours is a copy-book case of worn synchros. (First gear is also highly loaded, but since you only really shift up from first – hardly ever down to first on the move – its synchromesh rings don’t cop such a spanking.)

If the problem gets worse, you’ll probably find the upshift from first to second will also start to graunch. That’s when you’ll know for sure that the synchros need replacing. As you point out, though, to really diagnose this the fault requires an inspection of the parts and that means the gearbox out and pulled apart. While you’re in there, change the other synchromesh rings and the transmission’s bearings. And don’t put it back together without a new clutch.

In the meantime, however, if you’re prepared to shift very slowly and carefully while the vehicle is cold, you can actually drive around this problem. Many car owners have done so for years without further problems. Not best engineering practice, perhaps, but it’s a money saver.