If it is the exhaust manifold gasket starting to blow out then keep the driving down to an absolute minimum. You can go from sounding like a tractor to
sounding like an unmuffled freight truck in a moment. Depending on where the leak is, damage from excessive heat could result, including the head. Check for loose exhaust manifold bolts and expect possibly broken studs. Once you get a leak then tightening the nuts may temporarily improve things, but the gasket and studs should be properly replaced. Start hitting every exhaust bolt with a high quality penetrating oil like PB Blaster and repeat once or twice a day for a couple of days to make life easier for removal. If you have an EGR valve (down by the oil separator box) then go overboard soaking the manifold fitting and tell the mechanic to be careful not to bend the EGR tube to the manifold as it can get thin (esp. the left corner) and easily crack. It's no longer available from Volvo and would then need a weld repair.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now