5-Series (E12,E28, E34, E39, E60)Chat relating to the BMW 5-Series of all generations. Specific models include: BMW 518, BMW 520, BMW 520i, BMW 530i, BMW 528i, BMW 530i, BMW 518i, BMW 524d, BMW 525i, BMW 525e, BMW 528e, BMW 540i, BMW 535i, BMW 520d, BMW 525td, BMW 525d, BMW 530d, BMW 525i/xi, BMW 530i/xi. (BMW 5-Series Forum)
sucking noise you hear is indicative of an intake leak.
look under you hood and check your intake hoses for obvious cracking. In case, you can't see any visible signs, easiest way is to use a can of brake cleaner to aid you in your quest for finding the leak. spray around the intake area - AFM, rubber parts, especially where rubber joins to metal.
while you do that, notice the change in idle while you spray. There will be a marked change in idle the moment the brake cleaner soultion touches the break/crack
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