Please check both flex disks immediately! I had a rear one let go,
same noise, but severe vibration because a big chunk fell out.
ANY distortion, cracks, or signs of the inner cord being visible
means the flex disk is bad and failing. The rear one won't give you
too much trouble (mostly bad vibration followed by a no drive
condition), but if the front one comes apart, the now free front half
will beat the dickens out of the floor pan, possibly coming right
through. Not pretty.
Check also for excessive free play in the differential -- the drive
shaft should not rotate much before the half shafts do (ideally,
none, in fact). The spider gear carriers wear, allowing excessive
play and resultant clanks, but not usually vibration.
The driveshaft u-joint can go bad, but this usually sounds like a bad
muffler and will cause excessive vibration of the floor pan just
about level with the front seat backs were the carrier bearing is.
Carrier bearing can go bad, too -- again, not usually without
squealing, whisting, or roaring at the carrier bearing.
Also try to move the driveshaft laterally in the carrier mount --
this mount should be quite firm, so if you can move it more than say
1/8" with a firm pull, the rubber is shot. This will allow the shaft
to orbit around the centerline rather than spin true, resulting in
noise and vibration, but be intermittant. Can appear only on hard
acceleration, but go away on a stop or on coast. Once it starts it
usually continues until the driveshaft is a rest, though.
Peter
On Dec 29, 2008, at 4:41 PM, ernest breakfield wrote:
hi gang!
time for an invitation to play one of our least favorite games;
diagnosis of an intermittent noise via the internet. ;-*
the suspect: Helga; an '85 300D California car, with just over
170K miles on it that i've had for the last 50K+ miles.
scenario: trunk moderately full of Christmas goodies and
luggage, cruising I5 at around 70+MPH on a trip downstate last
week. about 3-4 hours out i started hearing a rhythmic noise that
sounded at first like a tire thumping, but there was no associated
vibration to be felt. this noise gradually became progressively
louder; enough that i pulled off to see if there might be something
stuck under the car flapping, or anything else visibly out of
sorts, but peeping and grabbing and shaking things didn't turn up
any likely culprits. after crawling around under the car for a bit
and sitting on the side of the road, i decided to move the normally
dependable beast to a better location, and lo! no noise whatsoever.
we decided to see how close to our destination we could get and
proceeded, only to not have the noise show back up in the next hour-
plus; just enough time to get us over the Grapevine before dark.
to make a long story short, this scenario repeated itself with
varied frequency throughout the next several hundred miles;
sometimes without any noises for hours, and other times coming back
in as little as 40 miles. sometimes once the noise starts it sounds
like a u-joint click-clunking as the vehicle comes to a slow, and
other times it makes almost no noise at all while coming down to a
stop.
once the noise shows up it may or may not worsen under load, and
may or may not worsen with speed. sometimes it goes away
immediately as soon as the vehicle comes to a stop, and other times
it seems to need to sit for a while. we ended up making a practice
of finding many excuses to stop on the premise of giving our dog
more places to sniff, and that seemed to keep the noise at bay
somewhat.
final note of concern was while driving a short errand around
town last night, a sharp "Clank!" was heard followed immediately by
something that sounded for all the world like a bolt falling out of
place; yet nothing has been found to appear out of place yet by
crawling around under the car. note of concern to me, this was the
first time there have been serious noises that weren't rhythmic,
and this showed up while the car had just started doing errands at
sub-35 MPH.
my impression is that this is something related to the driveline
based on the rhythm, but i'm not sure what i might need to look
for. driveline doesn't seem to have any undue play when twisted by
hand, there aren't any tears on the axle boots, and there doesn't
seem to be anything out of the ordinary underneath.
i *hate* intermittent problems. i also hate problems that i
can't repeat with the vehicle at rest, making it impossible to
accurately determine where the noise(s) come from.
any hints, tips, or clues as to what i might look for before i
have to just start addressing the problem shotgunning-style by just
replacing everything that moves down there?
cheers!
e
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