Knock-Knock – Who's There?

Trying to find odd noises, squeaks, rattles, and groans
on a moving vehicle can be absolutely frustrating.  There
are several different methods you can use to find these
noises, and even more bizarre results of those noises.  
Some mechanics prefer the old “bounce on the bumper”
and listen for the squeak method. Others opt for the
sophisticated microphones that can be placed anywhere
on the car, then the tech listens to a recording, or live
while driving the car.  But, it never hurts to have a
helping hand (ear) along for the ride, while you drive
and they listen. 

Nowadays, most mechanics use a stethoscope made
especially for the automotive field, but I'll still grab an
old screwdriver first... just a habit I guess,
you know, old school stuff.  Holding the tip against
the engine and the handle against your ear will still
work just as well as some of those modern devices. 
Sometimes a light tap with a small hammer can help find the source, or swinging a door open and closed will give you a clue, whatever seems appropriate.  It’s the creativeness and ingenuity of the diagnostic mechanic to find these bothersome unwanted clicks, rattles, and squeaks that still amazes me.

One of my all-time favorite noise safari adventures was on a car that had a tap-tap, knock-knock sound which would come and go.  It was from an old customer who just picked up a sweet deal on a used car.  As usual, those sweet deals usually involve some wacky problem that no one has solved yet. He said he couldn't reproduce the sound because he was never sure what was causing it.  Now it was my chance to try and find the source of this annoying noise.  Ok, sure why not, I’m up for a challenge; it's been a while since I've had to trace out a strange noise.  Besides, the shop was a bit slow, and I had the extra time to spend on it.

“The previous owner had it at several shops, and nobody has even come close to finding it,” the owner told me, “and I don't want to start throwing parts at it, unless you're pretty sure it's going to fix it.”

I'm not one to just slap parts on a car anyway, so at least the owner and I are on the same page.  Looks like it’s time for a test drive around the block.  Maybe a couple of times around the shop will do the trick, and see if I can recreate this unwanted sound. I went around and around...and around… nothing, not a sound, no creaks, no groans, no tap-tap, and no knock-knock coming from the car.  The owner decided to leave the car with me for a few days so I could drive it off and on, hoping after a few more test drives I might have an idea.

After numerous trips I did manage to find a way to create the tap-tap, knock-knock sound.  You needed to be almost through a 90 degree turn, with a slight bump during the turn, and be slightly heavy on the throttle.  It wasn’t easy and the conditions had to be just right, but I was able to repeat the noise almost every time.  Just to be sure, I had the owner come along with me to verify this was the exact noise he was hearing.

“That's it!” the owner of the car said enthusiastically, “What do you think it is?”

“I don't have any idea, yet.  But now that I can reproduce it, I have a way to narrow it down to the source,” I told him.

Another day went by, test drive after test drive, up and down off the lift, flashlights beaming in every direction under the car, still nothing.  It just didn’t add up.  Ok, it’s about time to start getting creative.  One of the guys in the shop volunteered to be a live hood ornament.  (Good, cause I ain’t doing it…)  He would hang out the window while I juiced the gas pedal and sped around a corner.  (Talk about strange diagnostic efforts, but ya gotta do… what ya gotta do.)  I'm glad we didn't have any cops close by. There's no doubt this was an entirely unsafe way of finding the noise. 

First, we tried hanging out the passenger window, then the rear windows, but that didn’t work.  He wanted to hang on to the hood and try it, but I had to draw the line on that one; that’s just a bit too far on the dangerous side.  So instead he sat on the driver’s window sill with his feet in my lap, while holding onto the top of the door and leaning forward towards the front bumper.   I set the car in motion, hanging onto his belt... “Here we go....!” I yelled out. 
As we rounded the corner I gave it the gas.  With the engine rev'd up, and him hanging on for dear life, the knock-knock-tap-tap was unmistakable.  He shouted, “I got it!  It's right here, it's on this side and behind the tire!”

We drove back to the shop (with him back in the car of course), excited that we narrowed it down to one location.  After a bit more head scratching we both decided it had to be coming from the inside of the fender area.  Now it's time to see (who’s) what’s there.  When we took the inner fender down there was a lot more in there than we expected to find.  Bolted to the fender support bracket and hanging from a nylon string was a large nut, along with a message written on the inside of the fender.  The message read, “Ha, Ha, how long did it take you to find me?”  Yea, we found the noise alright. Looks like some wise cracking bodyman is having a little fun with us.  Hmm, let me think about this… several drive tests later, one guy with his butt hanging out the window, while I have a hold of his belt, and gunning the engine on a hair pin curve… Oh yea… real F?#@? FUNNY!  

I've heard rumors that somebody tried this on a car while it was on the assembly line, but I kind of doubt that actually happened.  There are too many inspectors and too many people around who would probably notice it.  But, I could see it happening at a body shop.  I guess it was my turn to be the recipient of this old practical joke.

These days when it comes to finding squeaks, clicks, knocks, and other strange noises you can bet I'll look behind the inner fenders.  Ya got me once… but not twice.  You can call me paranoid, you can call me cautious, call it what you want...  I call it practical experience, and that ain’t no joke.

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