C1 1 L Gasoline laquered

Hello, I need people who also drive a C1. And I have the problem that I hear a cracker in the engine when I’m gasting under load. If I give gas when I’m idle you don’t hear anything. The cracker is still speed-dependent (frequency is increasing at higher speed). I was already in the workshop with it (an open dealer because I bought the car there) and there it was noticed that they filled in 5W 30 oil during the inspection, but according to the manual probably 15W 40 in. So a change of oil was made. However, this didn’t really change something on the clacker (does it last until the new oil is distributed?). Unfortunately, I don’t have any acquaintance who has a C1 to compare all the time. So I turn to you here. Do you know such noises? Should that be so? The mechanic in the workshop said, since this is a chain motor and no one with timing belts, just listen to it. But that seems quite Spanish to me. 7 and has now driven 72000 km. He has a Toyota engine with 3 cylinders, 50 kW and 998 cc displacement. Furthermore I have no idea of engine technology. Thanks already and best regards Jens