Hi after having an annoying rattle @ 2.5k revs today my heat sheild dropped off, just sat on top off the back box making even more noise.
So I got underneath the car dragged it off now all noises gone.Question is does this need replacing does it serve a purpose, know lots just remove heat sheilds and bin.
I would fit a new one.
There is a lot of heat coming off the back box and I’m not sure what the ignition point is of any underseal/paint/rubber that may be around that area.
Also, there is a risk of running over plastic bags, bottles etc while out and about, which could lodge on top of the back box and eventually start combusting.
Just my opinion though…
Hi - I had the same issue with an NC of mine a few years ago - in my case the heat shield was corroded and had dropped down on back box making a terrible noise.
Like you I ripped it off (I actually ripped off all the shields back to front as they were loose), fitted a sports exhaust and forgot about the heat shield. Ran it daily for 2 years no issues, summer and winter.
To me the they are a bit of a gimmick and they also cause the body to rust where the shields are attached due to the junction of to different metals, steel and aluminium.
I have also removed them from Merc’s as they all have the same rattly and damn expensive heat shields.
Cars never used to have them and unless you have a 5.0 supercharged or turbo charged engine in the mx5 the exhaust will never get hot enough to warrant them. Think about it - there are plenty of places under the car near the body and plastic bumper without a shield anyway.
If you do want to replace they are held on by 4 rivets, accessible from under the boot carpet, you might need to remove the rear section of the exhaust to clean up the inevitable rust where they touched the body.
Personally I wouldn’t bother, just enjoy the rattle free experience!
They do stop the contents of your boot heating up though!
Thanks think I will talk to the garage if a cheap repair probably put it back on seeing its still in tact…but if it sounds messy will probably leave it off never put anything in boot and longest journey I ever do is a 20 mile leisure drive once or twice a year.
Took it to the garage they told me no point putting it back on so will just leave it off for time been .
Now it’s off maybe worthwhile removing backbox and sorting any rust on underside of boot pan.
I found my boot pan rusted in places, cleaned it up and sprayed with ‘buzzweld rust encapulator’ , primed then stonechip top coat.
Good work. 
Nice…I sit every night thinking I’m going to do stuff like this to my car,then never do.
