Right, had a spare few hours after work this evening and can't hack the rattling / clunking anymore so time to change the rear control arms. Over speed ramps or uneven surface under no power the sound appears to be behind your head and consensus is that it's these that are the culprit.
I've seen them called Upper rear control arms and also track arms. Also called the tuning fork arm due to its shape. Anyway the part number is 986.331.143.06 (TRW Part number is JTC1186 and has 98633104307/1 on the box)
The ball end bush in the new ones would not move at all by hand but the old ones were soft as a soft thing, see pics. Time to swap was approx 2.5hours
So here's a brief how to, not to difficult but one trick helped immensely and might save someone a lot of cursing and swearing.
I'll assume you can jack the car and whip the wheel off. I slid the wheel under the rear suspension aluminum brace in case the jack slipped or collapsed. I know I should use axle stands but did one side at a time.
When car is safe take off the plastic cover, 2 x 10mm plastic nuts hold it in. Pull out from the top, clear one bolt at a time and it will pop right off.
Once off you will have a clear view of the arm. The tuning fork end is held onto the coffin arm by a bolt from below and nut on top behind the rear disc / hub. The ball end is a single bolt to the chassis at the other end. Its a reach in and I reckon the job would be a lot easier on a lift with the plastic panels underneath removed.
The wheel end is easy to loosen so I slackened the other end first, I did it this way as I wasn't sure I'd be able to loosen it. It's very tight so be prepared to swing out of it. Space is really limited with pipes etc so a good long ratchet socket is your best bet. When you get it loose undo the hub end.
Now here's the trick that I found made things a lot easier. As the car is jacked the rear suspension is dropped completely and this puts a big twist on the arm and the ball joint. So therefore the arm is in a "normal position" when wheels are on and suspension is loaded which makes sense.
I got a bottle jack and jack up the hub as per pics. This put the hub back in its normal condition and released a huge amount of twist.
With this twist out the chassis end is much easier to remove and once loose you can nearly unscrew the bolt by hand.
To put in the new one, keep the hub jacked up push the chassis end right in to open space and slide the fork end over the coffin arm. Then just slide it around so that the ball end slots in nice and handy and thread in the bolt. This should go a long way by hand. Stick the ratchet on it and take it up but leave it loose. Leaving the fork end free gives you lots of scope to manipulate the ball / bush end in and position it.
The fork end will not line up now so lower the jack holding the hub. When it's down the bolt will slot in. Hub might need a slight push towards the front but it should go in handy enough. Tighten everything up and torque to 160Nm if you can get a wrench in. My torque ratchet was too big to get access for the chassis bolt but the hub one is easy.
Hopefully that will make sense with the pics below.
Old and new, see why they get called "tuning fork arms"
Play in old bush v's new
Hub jacked up to relieved twist on chassis bush/ball joint
New one in, note twist when hub is hanging down
Hub end with bolt up from bottom
BTW - wear a hat unless you want a head full of grit and dirt from the wheel arches!!
Traco
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Older but no wiser.
2003 Boxster S, Seal Grey
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