The manhole covers in your garden are for the drains on your house or are connected to drains of your neighbours.
In more recent years builders have used cheaper mild steel covers rather than cast iron covers. Mild steel versions do have a sort of protective layer but given time go rusty and will flex a bit.
Best thing to do is lift them and see how sound they are, if not too rusty and not subject to xs weight just repaint them with hammerite paint.
If too rusty get a replacement at a builder merchants, if the frame is ok you can just replace the cover
Maurice
January 2011