m42 buzzing at 3000 rpm

[idickers]

My '91 318is has developed a metallic buzzing sound when accelerating through 3000 rpm. It sounds like metal vibrating, like a muffler with blown baffles, except that I have a new Bosal rear muffler, new Magnaflow catalytic converter section, and the noise is still there when I remove the rear muffler. Three years ago I also replaced the timing chain, rails and tensioner, along with the exhaust manifold gasket. I can't see any obvious interference between the exhaust and the body of the car that could be causing this buzzing. Any suggestions were to start hunting?

[oldmann]

Look at the heat shield that's next to the valve cover.
Mine was cracked, buzzed like crazy.

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[idickers]

Rats. I was hoping it was the heat shield, but that's not the problem. From the engine bay the motor sounds strong, it's just coming out the exhaust. Even with the rear muffler removed I can hear it, so I don't think it's the Bosal rear muffler section.

[bmwman91]

For your sake, I hope that it is just a heat shield. I assume that it is coming from the engine compartment?

I had the exact same thing back in 2011-2012. It was a crack in the timing case around the idler pulley. I discovered this because the entire casting broke off and let the whole timing assembly come loose and the engine was a big paperweight as a result. Sadly, the only way to check this is to pull the lower timing cover off, which involves the big 240 ft-lb crank bolt, and even then I am not sure if you would be able to see if the crack is present.

The one other thing I can think of, which is also a pain to deal with, is the top bolt on the driver's side timing chain guide rail. The top bolt has a tendency to strip the threads in the head if the guide rail was replaced with the updated part at some point but the accompanying longer bolt was not used (the original was 40mm long, the new guide requires a 45mm long one). The bolt head could be rubbing against the upper timing case cover. You can probably see enough with the valve cover removed to verify if this is the case. Again, letting this go for too long is very bad.

[dpmcdonald]

Timing chain tensioner?