Anyone else had this issue?

[911hillclimber]

I have a 73T with a stock 3.2 engine and Sports 915 box.
Everything works, engine strong, box good as it was rebuilt a few years ago, so the power train is good.

On the road, and for several years there is a very general 'gruffness' as you go down the road at reasonable speeds.
Of course, I cannot describe the sounds well enough, but the noises/grumbles do sound like they are in a cardboard box, nothing like a roll bar touching the gearbox mounting plate etc.

So, off with the wheels today to see if there is anything obvious.
Checked:
CV joints (4) and there is very very little play, maybe 0.5mm at the gearbox flange with the handbrake on, on both sides. Shafts float well and equal freedom. Thus, i do not think it is the CV(s) rumbling.
No CV clicking turning etc at slow parking speeds. They were all replaced about 6 years/ 3K miles ago. All cap head bolts tight.

Wheel bearings. The car has the later model aluminium rear arms and the bearing play is ZERO, both rock solid and noise-free. Thus not them grumbling either.

Checked all parts for clearances, everything that could touch has at least a 15mm clearance.

Now, the possible root of the matter, and the reason for this post:
Both heat exchangers are old SSi but i think made by Dansk. They are both a good 20 years old and came of a well used rally car. Both are a bit beaten by stone damage.
The silencer is very old, about 30 years, maybe 35 and is a 'works' 'Liestrikt' (sp) 3 outlet sports job that really should be replaced for the next MoT but is not holed. It is a good silence for the road when all working as a 2 in/1 out.

The OS heat exchanger is good but on tapping it with my hand sounds as if something is loose (close?) in the casing. There is NO play in the case-to-pipes, this rattle seems more obvious in the box leading up to the heater control box.

The NS exchanger does not have the same 'sound' when hit by hand, but sound hollow as would be expected.

All bolts etc of the exchangers to the engine are sound and no cracks.

To the best of my knowledge, there are no baffles inside the SSI exchangers.

Anyone else had similar issues of noise/harshness and reasoned it is the exchanger?

[Gary71]

Bit of gravel inside the heat exchanger?

[Sam]

I mothballed my 914 for a couple of years because the engine was making a horrific rattle and I couldn’t face the bill.

It was a bit of gravel in the fan.

[jwhillracer]

/\/\/\ What they say!
I would drop the system off, and see which bit rattles. Either gravel, or just as likely a 30 year old silencer falling apart inside.
Alternatively, and cheaper, buy - https://www.screwfix.com/p/3m-1100-37db ... airs/50346

JW

[911hillclimber]

Deffo not a grave piece rattling around, this is a deep rooted sound grumbling, bit like low level groaning down a drain pipe!
Takes the edge off the driving experience!

I have a near new Dansk SS 2 in 1 out silencer in the shed, so will prob swop it over and see.

This car is nearly too loud for a reasonable conversation..

[911hillclimber]

Just as these things snowball...
Wonder if part of the heater control valve may have broken and dropped down inside the heat exchanger?
Rather not take the heat exchanger off and so risking an exhaust stud shearing off.

[hot66]

Engine / gearbox mounts ?

[wildtexas]

I'm curious as to how you know what "low level groaning down a drain pipe!" Sounds like. I can't remember when I last experienced that......

[Bruce M]

Ignore it.

Sounds a bit like road noise from old tyres or a noisy driveline bearing/ component. Low frequency drone that appears when speed is above a certain point & doesn’t really vary in pitch?

[911hillclimber]

Groans down a drain pipe: miss spent youth!

Engine/box mounts all Porsche rubber original design, all about 10 years old when I removed the set of polybushes.

Ignore it?
Probalby the right way to go.
The car drives great, just a bit harsh, could be play in the box after all these years.
Going to swop out the silencer as it is easy to do and a I have one, and the lovely 2 in 3 out is getting very rusty.

This is a very good summary!

Sounds a bit like road noise from old tyres or a noisy driveline bearing/ component. Low frequency drone that appears when speed is above a certain point & doesn’t really vary in pitch?

[Nine One One]

Rear handbrake pads - spring come loose inside the hub or piece of grit in there? What happens if you pull the handbrake on slightly whilst driving?

[911hillclimber]

Good thought, will try that when out in the car again.
The handbrake shoes i know are 1973 originals, sail through the MoT.

Dansk silencer now on. Bit of a fiddle getting in in as it is slightly fatter than the 2 into 3 unit. All on and just back from a short run through the village.
Sounds different, but until I can give it a good run at speeds similar to last Tuesday's, then not sure.

The old silencer sure looks past it's Sell by Date.

The rumble was very muted, not like gravel trapped ina disc etc, far more muffled.

Chrome tip looks more classic: