EGR 'thrppp' noise

[Fffil]

For a reasonable time now (probably at least 1 year) I've had a strange noise from my EGR valve. I took it off a while ago (2 years maybe) and gave it a good clean out - it wasn't all that dirty which was good news, but ended up being much cleaner. It's possible it's from then that the noise started but I didn't notice it then. I've since taken the EGR off again more recently to check for correct operation etc with a neighbours vacuum pump, and it appears to work as it should (ie opens with vacuum). I've also checked (as far as I can) that other elements of the vacuum system are working fine and as far as I can tell, they are. I'll try to make a recording of the sound and upload it to here. I think, it is just the piston moving up and down and making a noise on the seals - it is a rhythmic and pulsing noise about 2-3, maybe 4 times a second and is definitely coming from the EGR valve (with the vacuum pipe disconnected - no noise). Might be connected, might not be, but my mpg has gone from around 52-54 down to around 45 (similar journeys weather temps etc, measured over the course of a year or so).