transmission hum gets louder as speed increases

Hi guys, I wonder if you can help me diagnose the humming noise thats just started about 10 days ago from the front of my 2006 UK spec Prius. The hum is not noticable at all at low speeds but as the speed increases it becomes noticeable (around 35 mph or so) and gets stronger or gradually louder as the speed increases. the hum is not related to the engine speed at all as a quick accelleration to increase the revs of the ICE does not change the hum frequency at all. The hum resembles the noisy meshing of gears to me (I have a mechanical background) and reminds me of when the drive shaft bearings of my old conventional car started to fail some years ago. There are no failure lights on the dash and the mpg seems to be more or less the same as before although its very cold here in the UK at present so my usual 56 mpg has dropped to about 52. Mileage is 56K. I intend to take it to the dealer this week end but would like your opinions first.

If you need any more info to help the diagnosis then please just ask.

Hope you can help.

 

The frequency of the hum is much higher than anything like a stone stuck in the tyre tread. More like a high frequency buzz than a clicking noise like a stone would give. I have checked the tyre pressures and they are all fine. The last visit to the dealer for a service was 5K ago where it had the 50K service, the brake pads replaced and disks skimmed. Its been absolutely fine since the service and the I only noticed the humming about 10 days ago (about 500 miles ago). No, not run over any trees or old ladies, hit the kerb etc, nothing out of the ordinary at all. The only reason I noticed it at all was because I had to turn up the CD louder than normal to mask what I thought was just road\tyre noise, but I travel the same roads week in and week out and thats why I know this noise is not normal. To me it sounds like something in the transmission, but I dont know, thats why I'm asking here, I just wondered if it was a known or common fault for the Prius.

 

Thanks for the help so far, heres todays update: I have checked the wheel bearings for play, clunking or any signs of physical wear using the tried and trusted method of grabbing the top of each front wheel in turn and rocking the car as violently as I can from side to side, the bearings are rock solid, no play at all.
I also tried shifting into neutral from a steady 50mph to see what happened and the hum still continued as before related to road speed (good news I suppose edthefox5 ? ). The hum does not change at all when the steering is moved slightly or even moderately left or right either (unlike the Mail delivey guys cars who posted above). Dont get me wrong, this is not a loud deafining hum, just a noticeable change from a couple of weeks ago. These are no warning lights or anything unusual indicated on the dash, lower fuel consumption, or battery charging activity. I will look up the posts from Bob Wilson as edthefox5 suggests to frighten me even more. Is a failing electric motor in the CVT covered under the 100k mile warranty?

 

Thanks bedrock8x. Obviously every bit of advice on any forum is used at my own risk, I cant sue you for that now can I! Any chance you could help me identify if the wheel bearings are suspect by letting me know your method? please! or anyone else?

 

Thanks edthefox5 for the advice above, its nice to know I can still get good advice from some forum members at least! After 25 yrs of working on front line military jet fighters I think I know the implications of "fooling around with this critical safety related item" as some might put it.

I will try the Temp difference test later and let you know the outcome, if thats negative then I will put life and limb at risk and jack the car wheel up (one at a time of course) to listen to the bearing noise.

Update: OK been out for the high speed run (10 miles with cruise set to 80 mph) and the results were pretty much as I expected; all 4 wheels the same at just above ambient (its 5 deg C here in UK at the moment). I put my fingers through the wheel spokes and felt as close to the hub as I could and not the disk or brake caliper. So I'm pretty doubtful about the wheel bearing theory, but I will do the jack up test at the week end just to make sure.

One more quick question though, when travelling along the road and then shifting into neuteal, do the MG1 and MG2 rotating parts stop rotating or change their rotational speed? The reason I ask is because when I do this it has no effect on the humming noise, it just continues as before.

 

Thanks Rusty, I will check the tyres in detail for any defects.