What on earth is this noise?

Post by **Pyoor_Kate** » Mon Jul 10, 2006 3:12 pm

Okay, so for various reasons my car got run at near max revs for about 20 minutes (probably 5.5ish k / 1275cc ital engine) in /3rd/(!) a month or two ago. Since then I've had this noise, which, well it's a bit difficult to describe. It's kind of:

wwaaAAHH wwaaAAHH wwaaAAHH wwaaAAHH

It happens between around 4 and 4.5- 5k rpm in 2nd (I rarely do that, but I wanted to test), 3rd and 4th. I've not yet tried to hear whether it happens out of gear - although I probably should.

It's pretty darn loud - but if you continue past it (yeah, I know, but I have to get to work) up to 5+k, then it goes again (at least, it does in 4th, I can't bring myself to push the car that hard in 3rd or 2nd (not again)).

I've been unable to place it, initally I thought it was gearbox, but I'm now not entirely sure. Any suggestions for what it could be... at all...? Or where to start looking? It's hard to place it being as it's only me in the car :-/

Post by **Peetee** » Mon Jul 10, 2006 4:47 pm

wwaaAAHH wwaaAAHH wwaaAAHH wwaaAAHH

At what frequency? four wahhs a second or much faster/slower?
Is it a dry screeching, with vibration, clear, muffled?
Are they seperate sounds or is the tone continuous with a rise and fall?

Post by **RogerRust** » Mon Jul 10, 2006 6:25 pm

You could record it on your mp3 - now there's a challange. We will all want to record our rattles for diagnosis

Post by **Onne** » Mon Jul 10, 2006 7:41 pm

diagnostic noise analysis.... hmm

Post by **Willie** » Mon Jul 10, 2006 11:44 pm

If I read you correctly the sound occurs at the same revs regardless of which
gear it is in which would rule out the gearbox etc. It appears to be a resonance in the engine at those particular revs. I had one car which gave a
regular 'thrumming' at high revs but,if you persisted and went through it the
thing smoothed out again. I could actually FEEL this thrumming but if you can't and can only HEAR it then,hopefully, it is something on the car vibrating
at those revs in sympathy with the engine?

[/i]

Post by **bmcecosse** » Tue Jul 11, 2006 7:26 am

May be the fan 'beating' with something else.

Post by **Peetee** » Tue Jul 11, 2006 8:33 am

Maybe it's two seperate noises that are pulsing in and out of phase..sort of ...ish {don't know the technical terms for this}.
Our kitchen extractor fan does this sometimes.

Post by **ColinP** » Tue Jul 11, 2006 9:27 am

Kate,

Are you sure it's not the two-tone siren from the Rally?

It does seem to be a resonance problem though - as it happens at engine speed - not car speed.

Does the 1275 have the engine stay? (should it?).

Colin

Post by **bmcecosse** » Tue Jul 11, 2006 10:12 am

The noises 'pulsing' together is the 'beating' I referred to above. That's the technical name!

Post by **Judge** » Tue Jul 11, 2006 10:37 am

Onne wrote:diagnostic noise analysis.... hmm

http://mfile.akamai.com/12834/wm2/muze. ... obj=v20328|

Sorry, I just couldn't resist it

Post by **Willie** » Tue Jul 11, 2006 12:13 pm

The two frequencies acting together is a good suggestion. It is a common
phenomenom in picture transmission technology whereby the frequency of
the scanning and the frequency of the revolutions act together and produce
a rogue frequency which appears across the picture as a random pattern
known as 'Morray pattern'.

Post by **bigginger** » Tue Jul 11, 2006 12:21 pm

"Moiré" pattern, to be pedantic, but somebody may want to look it up

Post by **Willie** » Tue Jul 11, 2006 12:34 pm

Correct Ginger, thinking of my diving days!!

Post by **bigginger** » Tue Jul 11, 2006 1:00 pm

Post by **Onne** » Tue Jul 11, 2006 5:06 pm

Judge wrote:
Onne wrote:diagnostic noise analysis.... hmm
http://mfile.akamai.com/12834/wm2/muze. ... obj=v20328|

Sorry, I just couldn't resist it

Jolly good fun this! Do you also have the full version?

Sorry of the off-topic hijack, back to Kate's noise now

Post by **Judge** » Tue Jul 11, 2006 5:23 pm

Sorry Onne, I couldn't find a full version. As noises can be notoriously difficult to identify, particularly over the internet, I thought this clip might raise a giggle

Post by **Welung666** » Tue Jul 11, 2006 6:09 pm

It certainly did here Bill

Post by **Pyoor_Kate** » Wed Jul 12, 2006 3:12 pm

At what frequency? four wahhs a second or much faster/slower?
Is it a dry screeching, with vibration, clear, muffled?
Are they seperate sounds or is the tone continuous with a rise and fall?

It's about 1 - 1.5 times a Hz, and the frequency, I think, remains the same regardless of speed (ooh, I'd not thought about that) but the volume rises as the revs increase, then there's a fairly sharp cut off where it's not happening any more.

It sounds like one sound, it doesn't change in any way except volume, I think (although I've done two very hard night shifts and my brain's a bit like stew at the moment)...

Post by **Willie** » Wed Jul 12, 2006 9:48 pm

Got to be a resonance induced when there is a certain vibration from the
engine i.e. at the offending revs. Going to be trial and error to eliminate though such as take up free play on clutch pedal ditto the brake pedal when
in the 'noise' condition, change the position of the heater flap, tie the bonnet
opening rod to something to tension it. Are all fittings on the bonnet cross strut secure? are the bonnet buffer rubbers actually in contact with the bonnet? You can play for hours!! I had an annoying vibration which was eventually traced to a worn pin in the bonnet hinge!