Something is tapping under my rear fender!
 

I am hearing a loud "tap tap tap tap" when under any acceleration. The noise is coming from under the rear passenger fender and sounds like someone tapping a screwdriver against a metal pipe.
No noise when coasting or breaking - only when accelerating forward or in reverse. The noise is at wheel speed.

I am thinking CV joint... does that sound right? Just repacked the other one two months ago too :(. Only 20K on them since new.

Tomorrow I'll jack her up and take a look.

Check for either debris in your tire tread or a puncture.

A nail, screw, or other debris may also cause tapping or ticking noises.

Cooper

Could be cv joint coming loose.

Broken head stud on exhaust side?

definitely check this.

Or a loose wheel...check the studs.

Thanks for the ideas. I'll check bolts and CV. Head studs were replaced a while back, so I don't think its that.

It is interesting that you say there is no noise while coasting or braking - wouldn't this eliminate a wheel/tire/cv joint problem? Given your information of the noise, my first thought would be a broken head stud on that side (as jpnovak mentioned).

If the noise follows the engine revs consider head studs.

Sounds like a playing card in the bicycle spokes.

Noise follows wheel speed, and only occurs when under acceleration - not while coasting or braking. No noises when I rev the engine in neutral. The sound occurs in reverse gear as well as forward gears.

I pulled the wheel off and carefully checked the boots - no sign of tearing or injury to the joint. When I rotate the rotor by hand there is no unusual sound or resistance.

I will continue poking around, but I'm not sure what to check next. I need to change the oil anyway, so I may take a look at the studs while I'm in there. They were replaced in a rebuild 52K back, and looked fine when I did valves 4 months ago.

Did you check the tires

I would check the engine mounts as well.

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All of the above comments are very possible causes of your tapping sound but most are treating the suspect as a mayor structural or mechanical failure, I would like to offer a possible alternate solution since something very similar happened to me. Before you check all these suggested here, do a check on the little things like loose cables, loose cable couplings, loose hoses and lines, brackets and any similar small objects inside the fender and engine bay as well as those close to the body shell underneath. As I found out after months of frustration and disassembling every suspension component as well as opening up the engine, a simple and rather small plastic cable coupling loosely hitting the inside of the fender made a very (emphasis on very) loud taping "CLANK" noise that filled the inside of the cabin and made me tremble every time it did.
Because it sometimes sounded as a repetition and sometimes as one "CLANK" I could not solve the mystery until I finished every other explanation and focused on the little things.
A tiny velcro tie-down fixed the whole thing in seconds.
Sometimes the very simple is overlooked.
Hope this helps.
Cheers
MD

Tires checked out and look good. Nothing poking out, and the tires are new and tread looks good.

I inspected the mounts and didn't see any issues with them. I replaced the mounts 1.5 weeks ago. I have driven it around 100 miles since the mount replacement, and the sound started suddenly - I'm sure it was not present after the mount fix.

I did some basic checking, but didn't do a detailed check. I will definitely do that tonight.

Safety-related question. Is there any danger to getting in the car, running the engine, and engaging the transmission while the back is on jacks? If not, I can test putting it into gear and maybe pinpoint the clicking location. I'm jacked up on the torsion bar ends.

That's more dangerous than I would do. The car moves about under acceleration, you'd just need one little slip on one stand and you've got a pretty dangerous situation. Better would be to go gown to your local dyno and have them do a few pulls. That way you can listen and watch close up, safely. You'll also get some useful info about your motor.

I ran it with the rear wheels off, so there was no slip/movement to worry about. I do like the idea of using a dyno, but am trying to avoid driving it or shop expense stuff. I'll ask around to see if one of the local guys has an connection that might help on that.

So I ran her on the jacks w/ no wheels in 1st, 2nd, and reverse. No bad noises while revving up to 3K. In 1st gear, I tried braking to the floor while applying gas to keep her from stalling. The noise did not appear even under those circumstances - just warm brakes.

My next guess is to put the wheels back on and check her on the ground again.

Is there any chance its just low oil? She was running a little low (bottom of the range) last week, and I didn't put more in as I was planning to change the oil this week anyway.

I poked around the engine bay some more, and found a loose wire with a large washer-thing on the end. Its a sensor for engine temperature that would normally be installed with one of the spark plugs going through it. It is possible that it was bouncing around - manually tapping it sounded about right. I was also about a finger's breadth low on oil, so I put in a half quart to hold me over until my oil change later this week.

I ran around and around the block in all gears, forward and backward, and the noise is gone. Hopefully that's all it was. I guess I'll know for sure next time she's really hot. The noise happened at the end of a long drive.