Knowing when to call a halt

I've mentioned that Fiona, my lovely Volvo XC60, developed a humming noise in May which has continued. The Service Manager at the local Volvo dealership has given me a lot of personal time on this. He came out with me for a test drive over a month ago, and agreed that there was indeed a hum between 40mph and 60mph. He wasn't sure quite what was causing it. He suggested carrying out the least-expensive possible fix: a fresh update of the engine-management software, and the software that controlled the variable operation of the rear differential. This was done, but with no effect. The hum was still there. Today the hum has been systematically tracked down to vibrating gearbox cables. This is not anything one might see from just looking underneath the car. At certain speeds the cables vibrated, producing the hum, which could then be heard inside the car.

Volvo had already received reports of this from other customers, and had come up with a fix of a kind: a vibration-damper (basically a weight) which inhibited the movement of the cables concerned. It sounds rather an ad hoc solution to me, but nevertheless this is the one and only official remedy, and Fiona has now had this damper fitted. The number of complaining customers hasn't been large, and the problem doesn't have the status of a fault requiring a general recall. So I've had to pay. The cost of time spent on tracking down the hum, plus the cost of fitting of the cable vibration damper, has amounted to £333 so far.

I asked questions, paid, and drove away. But on the way home it was clear that the hum was still present - albeit less than it was. You couldn't now say it was irritating. But it was still audible, at least if you knew it was there and were listening for it. If you didn't know, you might not hear anything special.

Where does this leave me? Let's take stock.

On the negative side
Fiona isn't as quiet as she used to be between 40mph and 60mph. 
It's really only a half-fix, despite the money spent.
There isn't any other official solution - I'm stuck with this.
I'm £333 down. All that's come out of my savings for 2015.

On the positive side
Fiona is still as quiet as ever below 40mph and above 60mph. 
The hum that remains can be lived with, and doesn't spoil her in my eyes.
The various tests made have established the absolutely glorious facts that Fiona doesn't have gearbox problems, dodgy wheel bearings, or a self-destructing rear differential. 
And that, therefore, no huge bills are looming.
I can go off caravanning, or dash off on any long journey, with complete peace of mind.

On balance, I'm going to call a halt, spend no more money, and live with the half-fix. Realistically, I don't think I can take this any further. Making a great fuss at the dealers will achieve nothing, if I've already had the official Volvo remedy. It'll be different if Volvo come up with a better fix, and agree that there should be a general recall on this humming-cable problem - I shall then go back and press for that fix, and some money back to boot!

But for now, I have a car that's been confirmed free of various Awful Problems that she might have had. That's marvellous.

It also occurs to me that Fiona's ageing exhaust system - still sound, but five and half years old - may be creating vibration and resonance and buzzes throughout the car. Who knows, when a new exhaust has to be fitted, Fiona may suddenly become a lot quieter.