My 01 S40 has started to have a low end rumble ever since the temps have gotten cold. It can be heard at idle when first started, then can be heard when accelerating (around 1k-2k rpms) until the car is fully warmed up. Haven't had a chance to get it on the lift to inspect it, just wanted to see if others have had a problem around 90k miles. It feels like it is coming from where the flex pipe is located.

 

yes its common for the flex pipe to fail on the Gen1 S40s. You can find replacement flex pipes to splice in ahead of the cat on Ebay for about $70 or you can opt to replace the whole cat with flexpipe for about $275 for the part (aftermarket Bosal - assuming you're not in CA). Note if you replace the cat, you will likely also want or need to replace the O2 sensors so that'll add another $200 for both.

 

Also, if it starts sounding like a sewing machine until it warms up then check the bolts on the exhaust manifold. I had one bolt missing and a couple loosen up and it sounded funny until warm when the gaps would close up.