A strange humming noise

If you have ever seen the short 1956 comedy film The Case of the Mukkinese Battle Horn - a big screen vehicle for The Goons, and according to Wikipedia unexpectedly popular at the time (see https://en.wikipedia.org/wiki/The_Case_of_the_Mukkinese_Battle_Horn) - you'll recall a scene where the police - on the trail of the stolen horn - are knocking on the door of elderly befuddled couple Minnie and Henry, and half-deaf Minnie is querulously asking Henry 'What's that strange knocking noise, Henry?', and Henry, also half-deaf, is saying to her 'I can't hear you, Min! There's a strange knocking noise!' Well, it's hilarious to watch, even if I haven't put over the comedy element very well. The point is, my normally quite quiet car Fiona - a big posh Volvo - is making a strange humming noise as she travels along . And while it doesn't by any means drown out all other sounds, it's unwelcome - and so far unexplained.

I first noticed it after her annual service last May, two weeks before setting off for Scotland. The service was combined with her annual MOT, and I therefore expected the dealer to pay particular attention to fault-finding. And faults were found - or at least recommendations were made to get certain things seen to, even though Fiona did pass her MOT. They included fitting new brake rear discs and pads (which I expected; it was done a week later), and replacing a leaking metal turbo pipe and associated rubber hoses (not expected, but dealt with a few weeks later, after I returned from Scotland). At the service itself, I'd had the automatic gearbox fluid changed and four new tyres fitted. I'd also had the tracking checked. I like Fiona to be in tip-top condition, with nothing neglected.

In the two weeks after the service, but before heading north on holiday, I became aware of a humming sound that cut in around 40 miles per hour, and faded at 60 miles per hour. I put it down to that leaking turbo. I hadn't noticed it before - cars make many little noises, for various reasons and in various combinations, and like Minnie and Henry my hearing isn't the best for distinguishing between them, nor for saying where they might be coming from. This humming sound was odd, but never completely irritating, and there was the prospect that it would disappear when the turbo pipe was replaced. So I dismissed it from my mind.

But it didn't go when the pipe was replaced. So what was it?

A temporarily loud auto gearbox? I wondered whether fresh, clean, runny fluid would make the gearbox seem louder than usual. Didn't it seem louder when last changed three years ago? Then gradually quieter, as the fluid thickened up over the following 34,000 miles? Hmm.

The new rear brakes and pads? But I'd never had a humming sound from disc brakes and pads before. And there was nothing scraping or metallic about it, as there might be if a pad were lightly touching a disc. It sounded like the hum a roof rack makes, when going fast enough for the slipstream to make it sing. (But there was no roof rack fitted, nor anything obviously catching the wind) Besides, it seemed to be coming from the front of the car, so far as I could tell.

It couldn't be the turbo pipe, or one of the struts holding it into position. The entire assembly was shielded by the gravel plate underneath the front of the engine. So far as I could see, nothing was in catching the slipstream and making a noise. And of course the pipe itself wasn't now leaking. In any case, enquiry at the dealers at the time of pipe replacement established that a leaking pipe whistled rather than hummed.

That left only the tyres, presumably the new front pair. They were Continental CrossContact UHPs. The dealer had chosen them. I looked them up on the Internet. Ah, high-performance tyres, designed to give excellent grip. They certainly had a chunky tread pattern:

In contrast, the same-priced rear pair were Continental EcoContact5s, also selected by the dealer, also high-performance, but with low rolling-resistance to last longer. The tread pattern was much less chunky:

I'd merely specified 'Continental tyres as before, please!' but Continental had introduced a new range and the dealer had needed to chose substitutes for the old set of tyres. I thought the choice intelligent - front tyres with great grip were highly desirable for a car-and-caravan combo - three tons of hurtling metal, after all. And even when not towing, I noticed that cornering and general handling were wonderfully assured with these new tyres.

But was noise the trade-off? Or were at least the front pair noisy - at a certain speeds - when the air trapped in those deep grippy grooves might get squeezed, so as to set up a siren effect? I wondered. Maybe they were quiet on most cars, but not on mine. Fiona had the weight of its big heavy five-cylinder 2.4 litre diesel engine pressing down on those front tyres...

If it were just a less-than-optimal tyre/car match, then I was willing to put up with the humming noise till the front tyres wore out and could be replaced. At least I'd be getting terrific road grip meanwhile.

But then, with the next caravan holiday approaching fast, I made the mistake of looking up 'humming sounds' on the Internet. I wish I hadn't! I read stories in various forums about hard-to-track-down sounds. Horror stories. Stories that suggested that my car was highly likely to suffer from shot wheel bearings and self-destruct differentials. That ignorant and untrustworthy garages fumbled about, and were taking owners for a ride. That Volvo were rubbish. And that, inevitably, it would cost megabucks to get these things fixed.

I began to feel that not only had I been a stupidly hubristic to treat myself in the last few months to new clothes, a new red handbag, a new digital radio, and lately a new digital camera; but that all my expenditure for the rest of the year was now in jeopardy. I'd need that money to get Fiona fixed. No more holidays; all kinds of economies. Oh dear!

Well, I slept on it. The best thing to do. And then yesterday morning, now in a better frame of mind entirely, I drove over to the Volvo dealer's new premises in East Worthing. I spoke to a chap I trusted. He came out to the car and we discussed what was going on. He was reassuring. He thought it could only be tyre noise. If I was worried, I could phone the following Monday, and go for a test drive with the Service Manager (another man I knew and trusted), so that he could listen to the hum and its behaviour, and assess what the cause was.

So that's what I'll do.

It's very like going to the doctor with a nagging complaint about some little pain, not accepting the initial assurance given, and wanting a second opinion even if it may lead on to inconvenient treatment.

Watch this space.