This is on a 94 C280. Checked the secondary air pump by energizing with 12V source. Makes a soft, low whirring noise. Definitely would not be heard over engine if the engine was started. The connection in the car to the pump is producing 12V (on cold start ignition). However, the fan is not making the high pitched whirring sound when the car is started cold. When applying the same external 12V source to my 96 C280 I get the same soft, low whirring noise. However, on cold start on the 96 there is the customary high pitched whir you can hear over the engine noise as the pump runs for the 90 or so seconds. On the 94, instead there appears to be a low, rumbling sound (possibly) from the air intake side. Could a clogged EGR recirc pipe cause the fan not to run at full speed and/or cause the rumbling noise and thus the P0411? Or could the air pump still be defective? The rumbling noise subsides after warmup.