'93 240 wagon, 140,000 mi. During acceleration from any speed, I get a rattling noise from the engine that is similar to spark knock (but it isn't). Noise disappears at cruise w/ constant throttle position. It comes back when accelerating. Could it be a rocker arm or timing belt??

Thanks,
Dave

If it turns out to be a hole rusted through the inner pipe of the downpipe from the exhaust manifold to the 'cat', keep in mind that some aftermarket pipes, and anything fabricated at your local muffler shop, will be a single-wall pipe, as opposed to the OEM double-wall pipe.

Using a single-wall pipe will have a moderately negative impact on the performance of your catalytic converter.

Although it is not at all likely, if your car requires state emission inspections, a sharp inspector who knows old Volvos might fail you if he sees a single-wall pipe down there.

No, it is definitely not a rocker arm and I have not heard of a timing belt noise but I suppose the tensioner might get noisy if dried out or incorrectly installed. You can have a look at the belt for frayed edges or running position on the pulleys. I do not really have a clue about the noise but it could be noisy valves (valve setting), water pump, AC compressor rattling, exhaust loose etc. Sorry, I can only rule out the rocker arm hypothesis for sure. You don't have them on a B230F.

Just had a look at the timing belt. Seems OK, no fraying, appears in good condition. Also, had a look under the valve cover. Everything looks fine. Could the valves need adjusting?

Dave

If it isn't a case of loose parts you might check your knock sensor.
When I got my wagon it had the same thing. I thought I needed to adjust the valves and put it off for later. The first time I removed my intake I found the problem.
The spade connection on the knock sensor was loose on its post. I resoldered it. After reassembly, no knock on acceleration.
My engine has a different sensor than yours, but might it be a similar problem?
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Al Hagenberger '84 245 310K, '87 744 172K

Most likely the heatshield around your down pipe. This connects to the exhaust manifold and connects to the cat. Mine was rattling and shot at about the same mileage. I opted to go with a header for not much more than the new downpipe. Take it to a reputable LOCAL muffler shop and they will diagnose any exhaust or intake leaks for free. It never hurts.
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92 244 lowered, Bilsteins, IPD sways, Unitek cam/header, TSW Revo 18x8's - 94 965 Niiice MONEY PIT! - 76 Datsun 280Z lowered, modified head, cam, headers, intake

Your assessment was right on. I found the heat shield coming loose - used a pipe clamp to secure and noise is gone!! I had the down pipe replaced a few years ago at MIDAS (lesson learned here!!!) and I bet it's single-walled. Never knew there was a difference. But, ever since it has been louder (exhaust noise) than it was when the car was new. Now I know why. Thanks for the info!