Oil them. The clips onto the soffit should allow the thermal expansion
of the gutters . Do not glue them to the gutter as the gutter will
want to expand, it can't if glued, so the gutter will buckle .
Lubricate the clip/gutter interface. The recent increase in volume
could be caused by the sun heating the gutter more (is it black because
it needs cleaning)or due to any previous lubrication becoming worn out
over time. Oil is being a bit flippant perhaps silicone or similar.

C.F. Stiction (violin bowing)

Brian Gaff wrote:
Now my plastic gutters have been up for some years, and I had them done by a
professional. OK they tend to crackle a little bit in the sun, but just
recently the crackling is more like loud cracks at variable intervals and if
I'm attempting to catch up on sleep during the day its very um awakening. Is
there anything I can do to quieten them?

Brian