There is something wrong with George's car. It was only recently that George had the bonnet up and now he's tinkering with it again. George had a play round with the engine the other day when he had no business being in there. Granted, it is his own car but he is doing more bad than good. It's all very well him changing a tyre but an engine is a completely different piece of machinery. It's obvious he thought that he'd have a play round to sort out that rattling sound and now it has got even worse. He has now decided to try and fix it and is out of his depth. This is why he deflects the question; he has no idea what he's doing.
So instead of answering, because he doesn't want anymore questions about cars as he knows nothing, he uses this opportunity to ask the age old question of why women are so interested in shopping? This is also a loaded question as he wants to Lynne to admit that it is her who has maxed out all his credit cards. He is scared of Lynne and is clearly nervous. The hand that is holding his shammy is shaking quite badly. Lynne has a nasty temper so he is being as passive aggressive as possible.
George's line of questioning has not worked. Instead of an admission of guilt, George has now inadvertently agreed to a trip to the shops. He also has to drive his clapped out old motor with a rattling engine. Lynne will not take kindly to being taken to a new boutique in a backfiring Mercedes. George has elected to say the word 'sigh' instead of actually sighing. Perhaps this is an ironic sigh. He is unhappy about Lynne spending all their money, but he is so tired he says sigh in order to again try and antagonise Lynne into a reaction. Unfortunately, Lynne only has one thing on her mind - a new frock.