Just to satisfy a curiousity i’ve had for several years now, and been too concerned about looking like a complete anorak to ask… (I’m past caring now!) The ‘knock’ heard when changing from high to low range and vice versa on a range-change gearbox, what is it exactly that makes the sound? I’ve drawn my own uneducated conclusion over the years that it’s a shaft of cogs moving in and out of position giving access to the new ratios, which has satisfied me til now, but knowing me i’m probably way out!
close enough for me.
if you got a 4 over 4, you actually have a four speed gear box, which goes into an overdrive (or a two gear gear box).
so you use 1,2,3,4, then change the overdrive into 2nd gear, then again use 1,2,3,4.
then the cluncky noise is the overdrive unit changing gear. presumably its loud so you know it has changed gear
Nip in the garage and ask to have a look at a gearbox with the top off, it will probably have the selector forks showing.
The noise (clunk) you hear is just the pneumatic actuator sliding a gear cog into its new position.
Now run along while I play with my baulk rings 
A memorable bit from Kellys Hero’s
Oddball: Hi, man.
Big Joe: What are you doing?
Oddball: I’m drinking wine and eating cheese, and catching some rays, you know.
Big Joe: What’s happening?
Oddball: Well, the tank’s broke and they’re trying to fix it.
Big Joe: Well, then, why the hell aren’t you up there helping them?
Oddball: [chuckles] I only ride 'em, I don’t know what makes 'em work.
Big Joe: Christ!
OR…
ROG: Hi, Boss.
Boss: What are you doing?
ROG: I’m drinking coffee and eating cheese, and catching some rays, you know.
Boss: What’s happening?
ROG: Well, the truck’s broke and they’re trying to fix it.
Boss: Well, then, why the hell aren’t you up there helping them?
ROG: [chuckles] I only drive 'em, I don’t know what makes 'em work.
Boss: Christ!
Well thanks for the explanation chaps, i guess i wasn’t too far out then?! Just something you hear many times a day but i’ve never really thought about finding out what it was. I’m fairly clued-up on the rest of the hisses and whirr’s, but that one was just beginning to trouble me As a follow-up then, why do Volvo (and probably the rest of manufacturers) demand that you pre-select the range whilst still in gear? I know 99% of the time you would anyway, but what harm does it do if you don’t?
Roger Breaker:
As a follow-up then, why do Volvo (and probably the rest of manufacturers) demand that you pre-select the range whilst still in gear? I know 99% of the time you would anyway, but what harm does it do if you don’t?
No harm but the changing of the range is slower so if it is not pre-selected, and the change of stick position is done too quickly, the range may not have changed in time or, in some circumstances, you can be in ‘no range at all’, as sort of ‘no-mans land’
I hope I said that correctly and it made sense 
…
ROG:
ROG: I’m drinking coffee
The time it takes our lot to sort any problems out i’d more likely actually be on the wine!
Perfect sense, thanks! Just common sense really then, when i see a little red sticker it just makes me think the gearbox’ll fly to bits if i don’t do what it says on it Hasn’t done in 12 years or more though i suppose. Yeah, get the ‘no mans land’ in the Daf CF sometimes on start-up as Daf’s select high range when the ignition is switched off, but occasionally only half-select low-range when you turn it on again! 
On something like a 13 speed fuller or even a twin splitter, selecting the next ratio a while before you were ready to change would be pressing the selectors against a moving part inside the gearbox. this could cause premature wear.
These gearboxes didnt change immediately that you “hit the switch” they only change when you break drive, ie, lift off the accelerator momentarily. That is the reason they were so fast to change.
The alternatives only change as you pass through the neutral plane so don’t really matter what position the switch is in prior to selecting the next gear