CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Strange intermittent knocking/tapping

I have a problem with my 2008 Saturn Astra hatchback with about 75,000 miles. So far this has stumped every mechanic I’ve taken it to. They all want to have the car for a few hours to drive it around and diagnose the problem… which I don’t want to do just yet.

Sometimes when I’m slowing down, I hear a quiet tapping/knocking coming from somewhere. The back right maybe? It’s very quick, almost like a card stuck in the spokes of a bike wheel.

The strange part, though, is that the frequency doesn’t change as I slow down. Whether I’m going 10 mph or 20, the frequency is exactly the same. Even stranger, sometimes it’s not even there.

Some other things that might be helpful: I have a manual. Pushing in the clutch doesn’t change the sound when it’s there. Neither does hitting the brakes. I did have a blowout on the highway about six months ago (the back right tire…which is why I first thought that’s where the sound was coming from), but the sound has only appeared in the last few weeks.

Maybe there’s something in your tire or hubcap?

If the frequency doesn’t change with speed, then it’s something that is hanging from your car that is knocking around - either a hose, or a loose body part, or something that’s not supposed to be there.

As for “leaving it for a few hours”, that’s pretty standard fare - you can’t expect a mechanic to diagnose it without driving it, and you can’t expect them to drop everything they’re working on just because you’ve come into their shop. If for some reason you are truly averse to leaving your vehicle for a short while, then you need to call and schedule an appointment to go on a ride-along with one of the technicians.