Blown plaster?

[jesteh]

Extension built two years ago - brick on outside, aerated block inside. Extension has been plastered inside with sand, cement & lime scratch coat and multi-finish top coat.

On one wall I have a hairline crack along the length of wall which keeps re-appearing after I fill it. When repairing it yet again I've noticed that the plaster 'creaks' as I push it - this doesn't take a lot of the pressure, just pushing it with my thumbs causes a creaking noise. It also sounds hollow when I tap it (but this is more or less throughout rather than in particular place - this could be down to nature of underlying blocks used?).

Does this mean the plaster's blown? There are no major cracks or issues anywhere after 2 years - in fact apart from few hairline cracks it looks perfect. Am I worrying about nothing or not?

 

[roughcaster]

Gently rub the back of your fingernails over the wall rather than tap it. You'll hear any "blown areas" much clearer that way.
In my opinion, I'd say it was blown, especially if you can hear it "creak" when you push on the plaster. Aerated blocks have a lot of suction which can cause problems with adhesion, especially render/scratch. It looses moisture/dries out quickly, due to the high suction of the blocks, before it has had a chance to set and cure, causing it to "blow". With this type of work, initial preparation is everything, and it can often take longer doing the prep correctly, than it takes to coat the wall. Lack of prep causes problems down the line.