Exhaust resonance - any ideas?

[Dinnov]

Hello All

Ive just finished a 3 year body-off rebuild and am thoroughly enjoying having my Griff back on the road again.

Only one slight annoyance (apart from having to run it in!) - I have a very loud engine resonance noise noticeable from idle to about 2500 rpm. It doesnt change pitch with engine speed, but just goes away at higher rpm. The noise is much like a wine glass resonating (no, I havent been drinking)- it makes my pride and joy sound like a road sweeper!

Im wondering if either the pre-cats, cat or silencer have become partially corroded / blocked while in storage? Could this noise be caused by excessive exhaust back-pressure?
I will be getting a de-cat Y piece shortly (any excuse!), so this might improve things.
But has anyone out there got any ideas?

Thanks.

[DonkeyApple]

Sometimes it can be down to how it is mounted as well?

[V8Bart]

Give it a few miles, it might soot up and stop doing it. I know it's not new but like you said it been sat for a while.

(sorry it might "soot up" un intended joke....ill get my coat)

[Dinnov]

Thanks for the response - its currently a standard exhaust system using original (15 year-old) mounts. Do you think I should try using new rubber mounts?

[DonkeyApple]

Not sure. Someone will know more. I had a car years ago which after a new exhaust developed an irritating boom. Transpired it was bad mountings that was holding it too firm in one place.

[Dinnov]

V8Bart said:

Give it a few miles, it might soot up and stop doing it. I know it's not new but like you said it been sat for a while.

(sorry it might "soot up" un intended joke....ill get my coat)

Thanks V8Bart - Ive done 300 miles to date, so its early days. I guess I could put more exhaust putty on the main downpipe to exhaust joint - I can hear it leaking slightly.

In the meantime, Im disciplining myself to do 1k miles before giving it a good blast - finding it very frustrating! Roll on the miles....

[The AJP Griff]

Before you go shelling out on a problem that didn't exist pre rebuild,I would check to see if the exhaust is touching the chassis somewhere. Be very scrupulous when you check it,as its difficult to check all the way through and the slightest touch can make the most annoying noise!

[Dinnov]

Thanks all for your suggestions. Today, I dropped the exhaust, loosened the rubber mounts, refitted it ensuring adequate clearance and re-sealed the front joint with putty. The noise remains. With the car on a ramp and the engine running, Ive now realised that this resonance sound is some form of induction noise - loudest next to the air filter. It must be coming out of the plenum.

Went for a drive this afternoon to add some running-in miles. Got home being followed by an oil slick from the gearbox. Quite disappointing, especially as the weathers so good.

Next job is to find and cure the gearbox leak.

Thanks again.

[davep]

I've had an audible 'plenum resonance' on my Griff from idle upto 2000rpm, cured by a small adjustment of base idle.

Edited by davep on Sunday 21st August 21:13

[clive f]

davep said:

I've had a 'plenum resonance' on my Griff from idle upto 2000rpm, cured by a small adjustment of base idle.

this...........happened on mine once and after resetting the idle it disappeared.

[CHGRIFF]

I had a similar problem, in my case the 2 exhaust tubes exiting the silencer were touching the chassis cross tube. I resolved it by moving the entire exhaust and mounting plate forward and placing spacer washers between the chassis and exhaust mounting plate to gain some clearance. The exhaust can also toch the longitudinal chassis tube adjacent to the bell housing if not set correctly. Worth checking these reas again to be sure.

[clive f]

knackered engine mounts will cause the exhaust to touch the chassis too.

[Dinnov]

Thanks again for your replies. Im tending towards induction noise at the moment, so I'll try Dave & Clives idea of adjusting the base idle. Thats after Ive put some oil back in the gearbox. I dropped the exhaust again this eve and found that the tail shaft seal has popped out. Hence the oil slick yesterday. Not sure why this should have happened - I remember having to coax it in with a mallet. I'll put it back with some Loctite retainer this time!

[Dinnov]

Finally made time to change the gearbox seal and removed the pre-cats at the same time! I adjusted the hex screw next to the throttle butterfly (base idle?) and my resonance noise disappeared. Thanks to Clive and all who contributed their ideas.