Ford Focus Rattling sound....

Hi,

Newbie here to the forums and first post.  I have a problem and hopefully, someone can help.

We bought a Ford Focus Titanium 2.0 Auto 2009 in December last year.  Two owners and 72k on the clock.  Only problems with it were the power steering, wouldn't lock on one side, battery and alternator.  All of this was put right with new parts and was going great.  A couple of months ago we noticed that the engine was getting louder and a loss of power.  Also a clanking sound from underneath (heatshield).  We took it to the garage which lucky enough we have a 2-year warranty with and picked it up the same day and it turned out it was the catalytic converter.  I told them about the problem with the clanking noise and they checked the heat shield and said it was firmly attached.  After picking the car up the same day with a new cc fitted the noise continued.

We contacted the garage again and booked another visit, which was yesterday.  We had a phone call to say the job had been a bit of nightmare and they couldn't locate the problem but the had partially removed the heat shield from the rear half of the car.  We picked it up last night but only went a hundred yards before we could hear it again.  On smooth roads, its fine but any dips, holes etc the problem starts.  It sounds like its coming from the left-hand front and rear passenger side and only noticeable when going over roads that aren't flat.  

Now I've looked on the net and possible causes and it's a minefield.  The garage said that they had checked everything underneath and couldn't find anything other than the heat shield.

So hopefully any ideas would be gratefully appreciated.

Cheers

Rob