Ford KA- Weird Noise

Hello, I have a 6 year old new style Ford KA. It has done 40,000 miles. For the last couple of months, on and off I have heard a noise sometimes when driving. At first I would turn the radio down, open the window and the noise was gone and I just thought it was imagination. In the last couple of months my car has been in the garage 4 times, and had a service and MOT within the last couple of weeks.
I really don't know much about cars, as I am sure you can tell! The noise, sounds like it is coming from the drivers side, I had heard it the last couple of days. it only happens when I have been driving for about 15 minutes, I always seems to hear it when I am on the same stretch of road each time. It sounds like a light rattle. As soon as I take my foot off the accelerator the noise stops. Then I carry on driving normally, the noise maybe carries on for a couple of seconds, but as soon as I turn off the noise stops, and I don't hear it again until I am driving home the next day. I have had a look around that area, but I don't really know what I am looking for.
Could anyone offer advice? I don't want an expensive trip to the garage if I can help it. Currently the only driver with two family members poorly and really need to keep working and driving daily!

Many thanks- Charlotte