I dismantled by front heater intake stuff a few weeks back to check and oil the fan motor. I found the heater matrix was black and clogged with strange stuff (growth? mildew?) that could not have been helping airflow at all and might well have explained the smell in the system that required periodic applications of Dettol spray disinfectant.
I picked it all out (well as much as I could, its a bit tricky as there's piping etc. in front of the matrix, and the aluminium fins are v. delicate so you have to be gentle) carefully with a pointed bradawl, hoovering with a pointed attachment and blowing with compressed air at varous times to help dislodge stuff.
I noticed what looked like the decayed remains of fabric among the debris and wondered if this was the remains of a thin fabric that had covered the matrix.
Job done and everything reassembled, the system blows much much better and - though its early days - I think I may have banished the tendency to pongy air. On the other hand, you can hear the air rushing through the matrix in a way you never could before, once the car is doing any forward speed (say 20mph or more). I think, though I can't be sure, the matrix might have had a free breathing fabric in front of it to act as a diffuser for the airflow and thus reduce noise (as well as protect the matrix from ingress of stuff/damage though that doesn't make much sense given that no-one is going to service that area and the material can rot over time, as we've seen).
So if you are doing yours, it might be worth seeing if you can fit a material in front of the matrix. I wondered about a free breathing nylon as being rot free.
Oh and I used some stick on draught excluder foam strip to replace the perished rubber seal where the motor housing mates with the matrix housing. No idea if that made any difference!
The customers tend to all look the same by ten o'clock! 
