Rattle / knock at 1500 RPM

I have a 2001 V6 Santa Fe which has travelled 100.000 ks. When the motor is at about 1500 rpm and not under load, it has a rattle or knock. At all other revs the motor is quiet. It doesnt use any oil,
has had the timing belt changed and is driven carefully and has been using Mobil 1 5w 30 for the last two years. Can anyone tell me what the noise might be and is it serious.
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It's hard to say without hearing the noise. You'll need to isolate the source of the noise. If you're unable to do this yourself, take it to a competent and reputable shop.
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