Exhaust seems to blow in Winter

Sounds like "pinking" to me. Timing slightly too advanced for the octane rating of the fuel and the temperature - so it goes away once the engine is warmed up.

Usual way to check for an exhaust leak is to block off the tailpipe with your hand. Cover it with a bit of rag if you don't like getting dirty. A leak can then be more easily heard; if it's not leaking, it should almost stall before it pushes your hand away.

I can think of no reason why an exhaust would only leak in the winter, and wonder if it had a leak that could be heard when driving, how it passes MOT's.

Chris

Actually, I think it does pink at other times, too. There is a rattling noise when accelerating before it's properly warmed up. I wonder if this is another specific symptom that only happens in Winter?

Trouble is, I see no way to change the timing, because AFAIK the ECU takes care of all that. I've wondered if the knock sensor is broken, although I get no management light or fault codes. I also wondered if one or both of the oxygen sensors needed replacing, although again - no codes, and I get good readings at MOT time.

Okay, I'll see if I can make it more audible by doing something like that.

Well, I was thinking maybe the metal was contracting (due to the cold) just enough to make it leak until it had warmed up. Or maybe the choke (assuming the ECU does something about the mixture when it's very cold) was somehow creating denser exhaust gases for a few minutes.

The MOT place I take it to (a local ATS, who just keep passing it every year for £30) also does exhausts, so I assume they'd be looking for a sales opportunity. They never mention it.

Be gentle with it while it is cold. Presumably it has hydraulic tappets so these might not work properly until the oil is properly warmed up.

My first thought would be a loose or cracked heat shield on the cat, or even loose catalytic labrynth inside the cat.

Sadly, no hydraulic tappets. Just shims.

I've sometimes wondered if the pinking noise I sometimes hear is the cat's insides rattling around. I've tried thumping it, but it sounds solid. I wonder if I can get my borescope in the oxygen sensor hole?

That sounds unnecessarily cruel.

Do you want a parliamentary petition written against you? ;-)

Cruel, but sadly necessary. At least it will bring the problem to the world's attention :-)