Well, the mystery deepens. I'm not saying the noise isn't the stereo. For it was indeed a Sony. I tried to follow the instructions on that page
@Dave_S linked too. But I didn't ave exactly the same menu options...
- Rotate the Control Dial until CAUT ALM displays, then press the Select button...
I had an item called CT BEEP which I turned off. But the bleepedy-bleep noise still happens when I turn off the ignition. Plus the weird noise that sounds like the fuel pump priming. Here's a short clip of it. First I turn the ignition off, then back on again. the 'pump' noise makes sense when turning the ignition on. But why would it happen when I turn the ignition off? It must be something else?
In other news, I've just been doing a bit more inspecting under the bonnet and discovered two nasty things.
1: The tranny fluid which looked quite reddish in the bright sunshine we had yesterday is actually a bit more on the brown side of things in the rather less dazzling light of today. Thankfully it's still clear though. No trace of milkshake. But it looks like a good ol' tranny flush and filter change is going to be at the top of my todo list...
I'd put it at about no.3 on this handy chart I found, although it didn't seem too opaque. Maybe I'm at level 2 1/2:
Another thing I discovered is that the truck has run into something at the front at some time. There are two metal struts which cross in front of the radiators and the lower one of them is slightly bent in. Though thankfully has stopped just short of the radiator. I didn't spot it yesterday as its hidden from above by the upper strut and, from below by the radiator if [like I was] you're lying underneath behind the radiator peering up at the underside of the engine bay. There's further evidence of this in the fact the plastic of the leftmost headlamp surround has its bracket broken. And that whole side of the front headlamp / grille section is held on with only one bolt and some cable ties.
It'll all fix up easy enough but it's very shoddy and I'm kind of annoyed at myself for not spotting it yesterday. But I was really concentrating so much on poking round the mechanical and chassis side of things, I only gave the bodywork and interior a very cursory glance.
In slightly more promising news, I shone a torch down the gap between the air con radiator and the main one and there was no embedded crud in there. The main radiator actually looked in quite good nick, although I'll probably replace it anyway. I also looked at the coolant and it was very clear. Previous owner has either diluted the antifreeze to a stupid degree or used pure glycol. Because I'm not aware of any off-the-shelf antifreezes that don't have dye in them. Another hint that I need to do a full scale fluids and filters change ASAP.
I'm starting to think this truck is running well in spite of what maintenance it's had, rather than because of it. Although, to be fair to the seller, I did get a bundle of receipts with it showing 'stuff' had been done. But it was mostly things like brakes, batteries and bushes. One of the receipts was for 'Digital Wheel Alignment' in 2018. So, if that was done right, then the wheels shouldn't be out of alignment so soon [that as regards the previously mentioned tendency to try and drift to the left, which I'm still hoping will turn out to be the mis-matched and differently worn tyres across the front axle].