If she has been stabled 24/7 and not had much exercise it could be something as simple as her needing to clear her throat.
If she doesn't make a noise normally I would be tempted to speak to a vet for advice.They may want to give her a check up and make sure she hasn't got a respiratory infection.
No she doesnt do it normally. Can roaring just develop like this? God Im so worried. Shes been out during the day in at night. Shes bedded on straw and haylage.
If this is new Penfold, then I think you need to talk to your vet about it. There's a possibility that she has just become unfit after the icy weather and she needs slowly bringing back to fitness. But if she is really gasping for breath, then you need to look at all the possible reasons that would make her struggle to get enough oxygen in to her body. Reasons might include a chest infection (is she coughing, snotty nose, other horses got colds?), some sort of anaemia (not enough red blood cells to carry oxygen round her body) or a heart problem (unable to pump blood efficiently). I certainly wouldn't panic - a nasty virus can do this to a horse but if it IS a virus you need to stop working her hard until she's shaken it off or you could create further problems x