Mechanical rythmic tapping.

[ADENUF]

Gday all,
I have this persistant tapping/knocking coming from under the dash,its definatly mechanical in nature and it starts from the moment i start the car sitting in the drive way ,it taps/knocks a little bit more than every second,it doesnt seem to increase in tapping speed as i press the acelerator and once im moving general road/engine noise drowns it out so i dont know if it stops(i assume it doesnt).As i slow down for the lights etc i start to hear it ,it can vary in volume from a quiet tapping to reasonably loud and also it can stop for a second or two in idle but kicks right back in again.
Ive been to a mechanic and he said (shugs shoulders) maybe its a vacuum valve?under the dash and not from the engine bay(thankfully)
any sugestions would be greatly appreciated.
steve
98 st 4.5L Gu wagon

[MudRunnerTD]

G'day mate.

Silly questions first....... Is the heater on? is the Fan running? Turn it off and see if you can still hear it.

Second. go and grab a length of 6mm to 8mm hose from your local bunnings, you know the clear stuff, (Garden hose will do but a little big) and stick one end in your ear, an ear plug in the other ear and poke the other end of your hose up an around in the dash, you will soon find exactly where the noise is coming from

The hose will act like a stethoscope and will also drown out the other noise. the end will be a microphone and lead you straight to it. Works great under the bonnet too looking for anything from an exhaust leak to a squeak or Knock.

[wildgu6]

Ccheck not pipes or cables are hitting along the firewall in the engine bay, had a similar noise and it was the battery cables for second battery hitting.


TAP TAP TAP MERRY XMAS TO ALL AND A SAFE NEW YEAR TO COME

[ADENUF]

Hey mudrunner,the first thing the mechanic did was fiddle with the aircon heater fan controls,these didnt make any difference.
and ive got 3 feet of garden hose i used on the engine to see if the noise was comming from there.when i get home from work ill shove the hose under the dash,see if i can pinpoint where its comming from , thnx

[ADENUF]

thnx wild,but its not anything lose moving around i belive the sound is definatly a mechanical knocking/tapping.thnx

[Robo]

Any accessories ?.
lots of loose wires under dash ?.
where abouts exactly do you think noise coming from ?.
where are you located ?.

you may simply find it's a loose screw/bolt and vibrating with engine, seeing as you mention it's faster than once per second.

I have a cable "that only in cold weather" tapps against something under the dash for a while untill cabin warms up.

[ADENUF]

Thnx for the reply Robo,the only accesory under the dash is the electric brake tow pack,thats to the left of my left knee,
no loose wires,all very tidy under there as far as i can see.
dificult to tell exactly where the noise is coming from sounds like its directly behind the radio area towards the fire wall.you can only hear it in the cab
not in the engine bay.
i dont think its a loose something as it sounds as i said before very much mechanical .to give you an idea at first i thought id damaged a piston or(as ive just resently had the tappets adjusted)and the mechanic did a dodgy job..tappet noise.its almost an-- excuse--( doonk doonk doonk sound)
as mudrunner sugested ill get up under the dash next couple of day with the hose and see if i can pinpoint it a little better than i have atm,
Do you know of any vacuum valves in that general area as the mechanic seemed to sugest?
hoping for a manual from the kids for xmas
steve

[stock patrol]

Thnx for the reply Robo,the only accesory under the dash is the electric brake tow pack,thats to the left of my left knee,
no loose wires,all very tidy under there as far as i can see.
dificult to tell exactly where the noise is coming from sounds like its directly behind the radio area towards the fire wall.you can only hear it in the cab
not in the engine bay.
i dont think its a loose something as it sounds as i said before very much mechanical .to give you an idea at first i thought id damaged a piston or(as ive just resently had the tappets adjusted)and the mechanic did a dodgy job..tappet noise.its almost an-- excuse--( doonk doonk doonk sound)
as mudrunner sugested ill get up under the dash next couple of day with the hose and see if i can pinpoint it a little better than i have atm,
Do you know of any vacuum valves in that general area as the mechanic seemed to sugest?
hoping for a manual from the kids for xmas
steve

If you think it might be a vacuum valve try to disconnect the main vacuum hose from under bonnet to isolate heater controls. Then see if noise still there.

[Robo]

Get your hands on a copy of nissan fast and have a look at parts pics for what's where for your GU.
I had a look my self and it appears to have vac actuators in about 3+ locations that I can see.
top left of fan box assembly.
top right of air box assembly.
bottom of air box assembly.
lower front of air box something else there also.
personally, I think the vac actuators are not the cause, but I cant hear it so, good luck.
cheers

[nissannewby]

Does it have the airbox that sits on top of the motor? If it does check it has been put back in place correctly and isnt touching the firewall. I have seen similar issues as the engine will move around a little being on rubber mounts.